[nrf noup] zephyr: boards: thingy53: remove multithreading config

nrf-squash! [nrf noup] boot: nrf53-specific customizations

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/thingy53_nrf5340_cpuapp.conf

@@ -31,7 +31,7 @@ CONFIG_NORDIC_QSPI_NOR=y
 CONFIG_NORDIC_QSPI_NOR_FLASH_LAYOUT_PAGE_SIZE=4096
 CONFIG_NORDIC_QSPI_NOR_STACK_WRITE_BUFFER_SIZE=16
 
-# Required by USB and QSPI
+# Required by USB
 CONFIG_MULTITHREADING=y
 
 # USB