Stop supporting DELTA_CONVERT_ICEBERG_UNSAFE_MOR_TABLE_ENABLE (#5017)

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## Description
Stop supporting `DELTA_CONVERT_ICEBERG_UNSAFE_MOR_TABLE_ENABLE`. It's an
extremely unsafe flag, which can lead to a corrupted table. As it is
strongly not recommended, this PR proposes to stop its support

## How was this patch tested?
UTs

Author: ChengJi-db

iceberg/src/main/scala/org/apache/spark/sql/delta/IcebergFileManifest.scala

@@ -19,7 +19,7 @@ package org.apache.spark.sql.delta.commands.convert
 import scala.collection.JavaConverters._
 import scala.collection.mutable
 
-import org.apache.spark.sql.delta.{DeltaColumnMapping, SerializableFileStatus}
+import org.apache.spark.sql.delta.{DeltaColumnMapping, DeltaLog, SerializableFileStatus, Snapshot => DeltaSnapshot}
 import org.apache.spark.sql.delta.DeltaErrors.cloneFromIcebergSourceWithPartitionEvolution
 import org.apache.spark.sql.delta.commands.convert.IcebergTable.ERR_MULTIPLE_PARTITION_SPECS
 import org.apache.spark.sql.delta.logging.DeltaLogKeys
@@ -100,14 +100,8 @@ class IcebergFileManifest(
       return spark.emptyDataset[ConvertTargetFile]
     }
 
-    // We do not support Iceberg Merge-On-Read using delete files and will by default
-    // throw errors when encountering them. This flag will bypass the check and ignore the delete
-    // files, and in other words, generating a CORRUPTED table. We keep this flag only for
-    // backward compatibility. No new use cases of this flag should be allowed.
-    val unsafeConvertMorTable =
-      spark.sessionState.conf.getConf(DeltaSQLConf.DELTA_CONVERT_ICEBERG_UNSAFE_MOR_TABLE_ENABLE)
     val hasMergeOnReadDeletionFiles = table.currentSnapshot().deleteManifests(table.io()).size() > 0
-    if (hasMergeOnReadDeletionFiles && !unsafeConvertMorTable) {
+    if (hasMergeOnReadDeletionFiles) {
       throw new UnsupportedOperationException(
         s"Cannot support convert Iceberg table with row-level deletes." +
           s"Please trigger an Iceberg compaction and retry the command.")
@@ -142,8 +136,7 @@ class IcebergFileManifest(
 
     val dataFiles = loadIcebergFiles()
 
-    dataFiles
-      .map { dataFile: DataFileWrapper =>
+    dataFiles.map { dataFile: DataFileWrapper =>
         if (shouldCheckPartitionEvolution) {
           IcebergFileManifest.validateLimitedPartitionEvolution(
             dataFile.specId,

iceberg/src/test/scala/org/apache/spark/sql/delta/ConvertIcebergToDeltaSuite.scala

@@ -862,15 +862,6 @@ trait ConvertIcebergToDeltaSuiteBase
       spark.sql(s"DELETE FROM $table WHERE id = 1")
       // By default, conversion should fail because it is unsafe.
       assertConversionFailed()
-      // Force escape should work
-      withSQLConf(DeltaSQLConf.DELTA_CONVERT_ICEBERG_UNSAFE_MOR_TABLE_ENABLE.key -> "true") {
-        convert(s"iceberg.`$tablePath`")
-        // ... but with data duplication
-        checkAnswer(
-          spark.read.format("delta").load(tablePath),
-          (0 until 100).map(i => Row(i.toLong, s"name_$i"))
-        )
-      }
     }
 
     // --- UPDATE

spark/src/main/scala/org/apache/spark/sql/delta/commands/convert/ConvertUtils.scala

@@ -224,8 +224,15 @@ trait ConvertUtilsBase extends DeltaLogging {
       fs.makeQualified(path).toUri.toString
     }
 
-    AddFile(pathStrForAddFile, partition, file.length, file.modificationTime, dataChange = true,
-      stats = targetFile.stats.orNull)
+
+    AddFile(
+      pathStrForAddFile,
+      partition,
+      file.length,
+      file.modificationTime,
+      dataChange = true,
+      stats = targetFile.stats.orNull
+    )
   }
 
   /**

spark/src/main/scala/org/apache/spark/sql/delta/commands/convert/interfaces.scala

@@ -20,7 +20,7 @@ import java.io.Closeable
 
 import scala.collection.JavaConverters._
 
-import org.apache.spark.sql.delta.{DeltaColumnMappingMode, NoMapping, SerializableFileStatus}
+import org.apache.spark.sql.delta.{DeltaColumnMappingMode, DeltaLog, NoMapping, SerializableFileStatus}
 
 import org.apache.spark.sql.Dataset
 import org.apache.spark.sql.functions.sum
@@ -95,4 +95,5 @@ case class ConvertTargetFile(
   fileStatus: SerializableFileStatus,
   partitionValues: Option[Map[String, String]] = None,
   parquetSchemaDDL: Option[String] = None,
-  stats: Option[String] = None) extends Serializable
+  stats: Option[String] = None
+) extends Serializable

spark/src/main/scala/org/apache/spark/sql/delta/sources/DeltaSQLConf.scala

@@ -1707,14 +1707,6 @@ trait DeltaSQLConfBase {
       .booleanConf
       .createWithDefault(true)
 
-  val DELTA_CONVERT_ICEBERG_UNSAFE_MOR_TABLE_ENABLE =
-    buildConf("convert.iceberg.unsafeConvertMorTable.enabled")
-      .doc("If enabled, iceberg merge-on-read tables can be unsafely converted by ignoring " +
-        "deletion files. This could cause data duplication and is strongly not recommended.")
-      .internal()
-      .booleanConf
-      .createWithDefault(false)
-
   val DELTA_CONVERT_ICEBERG_CAST_TIME_TYPE = {
     buildConf("convert.iceberg.castTimeType")
       .internal()