jfs: remove unnecessary dentry_unhash from rmdir, dir rename

jfs does not have problems with references to unlinked directories.

CC: Dave Kleikamp <shaggy@kernel.org>
CC: jfs-discussion@lists.sourceforge.net
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>