jffs2: remove unnecessary dentry_unhash from rmdir, dir rename

jffs2 does not have problems with references to unlinked directories.

CC: David Woodhouse <dwmw2@infradead.org>
CC: linux-mtd@lists.infradead.org
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>