ocfs2: remove unnecessary dentry_unhash on rmdir/rename_dir

Ocfs2 has no issues with lingering references to unlinked directory inodes.

CC: Mark Fasheh <mfasheh@suse.com>
CC: ocfs2-devel@oss.oracle.com
Acked-by: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>