[nrf noup] boards: nordic: Remove nrf54l20pdk

nrf-squash! [nrf noup] dts: Select SoftDevice Controller DTS binding as default

nrf54l20_cpuapp_common.dtsi file was added unnecessarily
in this [nrf noup] commit
e9c0ac1

Target nrf54l20pdk has been replaced with target nrf54lm20dk.

Signed-off-by: Sebastian Głąb <[email protected]>

Author: nordic-segl