cmake: remove ADDITIONAL_MAKE_CLEAN_FILES for CMake <3.15

Zephyr now requires CMake 3.20 or newer.

Let's remove comment regarding ADDITIONAL_CLEAN_FILES only supported
with CMake 3.15 or newer.

Also remove the ADDITIONAL_MAKE_CLEAN_FILES which provided the same
functionality, but only for Makefiles when using CMake <3.15.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>