m68k: mcfmmu: remove stale part of comment about steal_context

The comment about steal_context() came from powerpc and a part of it
addresses differences between powerpc variants that are not really
relevant to m68k.

Remove that part of the comment.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>