[nrf noup] zephyr: boards: thingy91x: remove multithreading config

nrf-squash! [nrf noup] boards: Thingy:91 X release config

This is a follow up to the upstream PR #2375 and it removes
multithreading configs in 'noup' changes that were only here because
of SPI/QSPI use.

Signed-off-by: Michal Kozikowski <[email protected]>

Author: nordic-mik7

boot/zephyr/boards/thingy91x_nrf9151.conf

@@ -5,7 +5,6 @@ CONFIG_SPI=y
 CONFIG_SPI_NOR=y
 CONFIG_SPI_NOR_FLASH_LAYOUT_PAGE_SIZE=4096
 CONFIG_SPI_NOR_SFDP_DEVICETREE=y
-CONFIG_MULTITHREADING=y
 
 # Disable Zephyr console and use UART for MCUboot serial recovery instead
 CONFIG_CONSOLE=n