ceph: remove useless CEPHFS_FEATURES_CLIENT_REQUIRED

This macro was added but never be used. And check the ceph code
there has another CEPHFS_FEATURES_MDS_REQUIRED but always be empty.

We should clean up all this related code, which make no sense but
introducing confusion.

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