hpfs: remove unnecessary dentry_unhash on rmdir, dir rename

Hpfs has no problems with references to unlinked directories.

We leave one dentry_unhash call in place, in hpfs_unlink's strange path
where it tries to truncate a file because the disk is full.  I'm not sure
what the full story is there.

CC: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>