Update the default memory maps for `nrf54h20dk` and `nrf9280pdk` to
remove the `shared_ram20_region` and `shared_ram3x_region` nodes,
because their child nodes no longer need to be grouped together:
* IPC buffers in RAM20 are statically allocated.
* DMA buffers in RAM3x have separate access owners.
Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>