Remove the mmap_free callback from struct videobuf_qtype_ops.

All implementations of this callback do the same trivial check: return
-EBUSY if any buffer is mmapped. That can also be tested in the
videobuf core.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>