ceph: remove unused CEPH_MDS_LEASE_RELEASE related code

The ceph_mdsc_lease_release() has been removed by commit 8aa152c7
(ceph: remove ceph_mdsc_lease_release). ceph_mdsc_lease_send_msg will
never be called with CEPH_MDS_LEASE_RELEASE.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>