[nrf fromtree] tests: drivers: uart: uart_mix_fifo_poll: 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 8791fcf)

Author: nordic-segl

tests/drivers/uart/uart_mix_fifo_poll/boards/nrf54l20pdk_nrf54l20_cpuapp.overlay

@@ -5,20 +5,19 @@ common:
   depends_on: gpio
   harness: ztest
   platform_allow:
+    - nrf52_bsim
     - nrf52840dk/nrf52840
     - nrf9160dk/nrf9160
     - nrf5340dk/nrf5340/cpuapp
     - nrf5340bsim/nrf5340/cpuapp
+    - nrf54h20dk/nrf54h20/cpuapp
+    - nrf54h20dk/nrf54h20/cpurad
     - nrf54l09pdk/nrf54l09/cpuapp
     - nrf54l15dk/nrf54l15/cpuapp
-    - nrf54l20pdk/nrf54l20/cpuapp
+    - nrf54l15bsim/nrf54l15/cpuapp
     - nrf54lm20dk/nrf54lm20a/cpuapp
     - nrf54lm20bsim/nrf54lm20a/cpuapp
-    - nrf54l15bsim/nrf54l15/cpuapp
-    - nrf54h20dk/nrf54h20/cpuapp
-    - nrf54h20dk/nrf54h20/cpurad
     - ophelia4ev/nrf54l15/cpuapp
-    - nrf52_bsim
   integration_platforms:
     - nrf52840dk/nrf52840
   harness_config: