[nrf fromtree] tests: drivers: i2s: i2s_speed: 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 796adb6)

Author: nordic-segl