m68k: Remove unused set_clock_mmss() helpers

Commit 397ac99c ("m68k: remove dead timer code") removed set_rtc_mmss()
because it was unused in 2012. However, this was itself the only user of the
mach_set_clock_mmss() callback and the many implementations of that callback,
which are equally unused.

This removes all of those as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>