drivers: mspi: flash: remove no_multithreading test

nrf54h20 needs multithreading because the mram driver depends on it.
Remove this test as MULTITHREADING should be selected for the nrf54h20
and disabling it will cause a dependency loop fault.

Signed-off-by: Riadh Ghaddab <riadh.ghaddab@nordicsemi.no>

Author: rghaddab

tests/drivers/mspi/flash/testcase.yaml

@@ -36,9 +36,3 @@ tests:
       - nrf54h20dk/nrf54h20/cpuapp
     extra_configs:
       - CONFIG_MSPI_DW_HANDLE_FIFOS_IN_SYSTEM_WORKQUEUE=y
-  drivers.mspi.flash.mspi_nor_no_multithreading:
-    filter: dt_alias_exists("mspi0") and CONFIG_FLASH_MSPI_NOR
-    platform_allow:
-      - nrf54h20dk/nrf54h20/cpuapp
-    extra_configs:
-      - CONFIG_MULTITHREADING=n