cifs: remove unnecessary dentry_unhash on rmdir/rename_dir

Cifs has no problems with lingering references to unlinked directory
inodes.

CC: Steve French <sfrench@samba.org>
CC: linux-cifs@vger.kernel.org
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>