omfs: remove unnecessary dentry_unhash on rmdir, dir rneame

omfs does not have problems with references to unlinked directories.

CC: Bob Copeland <me@bobcopeland.com>
CC: linux-karma-devel@lists.sourceforge.net
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>