[nrf fromlist] samples: drivers: audio: dmic: 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

samples/drivers/audio/dmic/boards/nrf54l20pdk_nrf54l20_cpuapp.overlay

@@ -4,6 +4,10 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+/* Test requires loopback between P1.23 and P1.24.
+ * For best performance, PDM_CLK shall be on 'Clock pin'.
+ */
+
 &pinctrl {
 	pdm20_default_alt: pdm20_default_alt {
 		group1 {