tests: drivers: spi: drop non existing board

nrf54l20pdk has been dropped in 878ddbe.

Signed-off-by: Fabio Baltieri <[email protected]>

Author: fabiobaltieri

tests/drivers/spi/spi_controller_peripheral/testcase.yaml

@@ -115,5 +115,4 @@ tests:
       - nrf54h20dk/nrf54h20/cpuapp
       - nrf54h20dk/nrf54h20/cpurad
       - nrf54h20dk/nrf54h20/cpuppr
-      - nrf54l20pdk/nrf54l20/cpuapp
       - ophelia4ev/nrf54l15/cpuapp