m68k: removed unused "mach_get_ss"

The m68k machine helper function "mach_get_ss" function pointer is
set for some machines, but ultimately never used anywhere. Remove it.

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