[nrf fromtree] tests: drivers: retianed_mem: api: Cleanup after nRF54LM20 rename

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

Signed-off-by: Sebastian Głąb <[email protected]>
(cherry picked from commit cb3a94a)

Author: nordic-segl

tests/drivers/retained_mem/api/boards/nrf54l20pdk_nrf54l20_cpuapp.conf

@@ -14,7 +14,6 @@ tests:
       - nrf54l15dk/nrf54l05/cpuapp
       - nrf54l15dk/nrf54l10/cpuapp
       - nrf54l15dk/nrf54l15/cpuapp
-      - nrf54l20pdk/nrf54l20/cpuapp
       - nrf54lm20dk/nrf54lm20a/cpuapp
       - ophelia4ev/nrf54l15/cpuapp
     integration_platforms: