ocfs2: Remove fs dependency on ocfs2_heartbeat module

Now that the dlm exposes domain information to us, we don't need generic
node up / node down callbacks. And since the DLM is only telling us when a
node goes down unexpectedly, we no longer need to optimize away node down
callbacks via the umount map.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>