UBIFS: remove a bit of unneeded code

UBIFS: remove a bit of unneeded code

This is a clean-up patch which:

1. Removes explicite 'hrtimer_cancel()' after 'ubifs_wbuf_sync()' in
   'ubifs_remount_ro()', because the timers will be canceled by
   'ubifs_wbuf_sync()', no need to cancel them for the second time.
2. Remove "if (c->jheads)" check from 'ubifs_put_super()', because
   at journal heads must always be allocated there, since we checked
   earlier that we were mounted R/W, and the olny situation when
   journal heads are not allocated is when mounter or re-mounted R/O.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>