hostfs: remove unnecessary dentry_unhash on rmdir, dir rename

hostfs does not have problems with references to unlinked directories.

CC: Jeff Dike <jdike@addtoit.com>
CC: Richard Weinberger <richard@nod.at>
CC: user-mode-linux-devel@lists.sourceforge.net
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>