ceph: remove dentry_lru file from debugfs

The file shows all dentries in cephfs mount. It's not very useful.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>