[nrf fromlist] tests: subsys: fs: fcb: Cleanup after nRF54LM20 rename

Remove duplicated test configuration after
nrf54l20pdk/nrf54l20/cpuxxx was renamed to
nrf54lm20dk/nrf54lm20a/cpuxxx.

Upstream PR #: 92901

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>

Author: nordic-segl

tests/subsys/fs/fcb/testcase.yaml

@@ -14,7 +14,6 @@ tests:
     platform_allow:
       - nrf54l09pdk/nrf54l09/cpuapp
       - nrf54l15dk/nrf54l15/cpuapp
-      - nrf54l20pdk/nrf54l20/cpuapp
       - nrf54lm20dk/nrf54lm20a/cpuapp
       - ophelia4ev/nrf54l15/cpuapp
       - native_sim