ceph: use generic_delete_inode() for ->drop_inode

ceph_drop_inode() implementation is not any different from the generic
function, thus there's no point in keeping it around.

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>