[nrf fromlist] samples: drivers: watchdog: Cleanup after nRF54LM20 rename

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

Upstream PR #: 92432

Signed-off-by: Sebastian Głąb <[email protected]>

Author: nordic-segl