soc: nordic_nrf: Remove unnecessary inclusions

This is a follow-up to commit 4be102f8e05a9fa12290d8209a8cb38569265478.

Inclusion of `<system_nrf*.>` from `soc.c` files is no longer needed
since `SystemInit()` is not called from those files.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>