tests: drivers: flash: common: Cleanup after nRF54LM20 rename

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

Enable external memory node.

Use separate test configuration as external memory works
only on nRF54LM20 DKs with `external_flash` fixture.

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