[nrf fromlist] tests: lwm2m: Remove qemu-cortex-m3

This board just don't have enough RAM for all tests
so it is just a maintenance headache without any
benefits.

Upstream PR: zephyrproject-rtos/zephyr#79261

Signed-off-by: Seppo Takalo <[email protected]>

Author: SeppoTakalo

tests/net/lib/lwm2m/interop/boards/qemu_cortex_m3.conf

@@ -9,7 +9,6 @@ tests:
       - native_sim
     platform_allow:
       - native_sim
-      - qemu_cortex_m3
       - qemu_x86
     tags:
       - testing