hammer2 - Remove the hidden directory, rework deletions

hammer2 - Remove the hidden directory, rework deletions

* Now that inodes are separately indexed we no longer need the hidden
  directory abstraction to handle unlinked-but-open files.  Get rid of
  ALL the hidden directory handling code.

* Rework xop_unlink and hammer2_inode_unlink_finisher().  We cannot safely
  reference the inode chain's inode data to get the nlinks count.  Instead,
  figure it all out on the frontend using the active nlinks in the
  hammer2_inode_t structure.

* Fixes hardlink removal and rename issues.