media: mem2mem: Remove unused v4l2_m2m_ops .lock/.unlock

Commit f1a81afc ("[media] m2m: fix bad unlock balance")
removed the last use of v4l2_m2m_ops.lock and
v4l2_m2m_ops.unlock hooks. They are not actually
used anymore. Remove them.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>