applications: nrf_desktop: optimize MCUboot QSPI build for nRF52840 DK

Disabled the redundant Kconfig option (CONFIG_MULTITHREADING) in the
MCUboot image configuration for the nRF52840 DK and the MCUboot QSPI
build type.

Signed-off-by: Kamil Piszczek <[email protected]>

Author: kapi-no

applications/nrf_desktop/configuration/nrf52840dk_nrf52840/images/mcuboot/prj_mcuboot_qspi.conf

@@ -17,9 +17,6 @@ CONFIG_BOOT_BOOTSTRAP=n
 
 CONFIG_FLASH=y
 
-# Required by QSPI
-CONFIG_MULTITHREADING=y
-
 CONFIG_NORDIC_QSPI_NOR=y
 CONFIG_NORDIC_QSPI_NOR_FLASH_LAYOUT_PAGE_SIZE=4096
 CONFIG_BOOT_MAX_IMG_SECTORS=2048