m68k/sun3: Remove unselectable code in prom_init()

This was copied from SPARC, but isn't relevant for the supported Sun-3
models.

[Geert] Also remove the related extern declarations, and update the
	comment about prom_init().

Reported-by: Sarah Nadi <snadi@uwaterloo.ca>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>