fat: remove unnecessary dentry_unhash on rmdir, dir rename

fat does not have problems with references to unlinked directories.

CC: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>