tests: drivers: adc: adc_error_cases: 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]>

Author: nordic-segl

tests/drivers/adc/adc_error_cases/boards/nrf54l20pdk_nrf54l20_cpuapp.overlay

@@ -9,7 +9,6 @@ tests:
     platform_allow:
       - nrf52840dk/nrf52840
       - nrf54l15dk/nrf54l15/cpuapp
-      - nrf54l20pdk/nrf54l20/cpuapp
       - nrf54lm20dk/nrf54lm20a/cpuapp
       - nrf54h20dk/nrf54h20/cpuapp
       - ophelia4ev/nrf54l15/cpuapp