ext3: remove unnecessary dentry_unhash on rmdir/rename_dir

ext3 has no problems with lingering references to unlinked directory
inodes.

CC: Jan Kara <jack@suse.cz>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Andreas Dilger <adilger.kernel@dilger.ca>
CC: linux-ext4@vger.kernel.org
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>