[tune] New persistence mode cleanup: Delete legacy utilities (air.remote_storage, etc.) (#40207)

This PR removes some legacy utilities, including `air._internal.remote_storage`, `TrainableUtil`, and more.

---------

Signed-off-by: Justin Yu <[email protected]>

Author: justinvyu

python/ray/air/BUILD

@@ -124,14 +124,6 @@ py_test(
     deps = [":ml_lib"]
 )
 
-py_test(
-    name = "test_remote_storage",
-    size = "small",
-    srcs = ["tests/test_remote_storage.py"],
-    tags = ["team:ml", "exclusive"],
-    deps = [":ml_lib"]
-)
-
 py_test(
     name = "test_remote_storage_hdfs",
     size = "small",