sys/util.h: remove deprecated MACRO_MAP()

Since this is an experimental API and MACRO_MAP() was deprecated in
favor of FOR_EACH() in zephyr v2.3.0, we are within our rights to just
remove it without notice now. Do so and mention it in the release
notes.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>