ocfs2: use system_wq instead of ocfs2_quota_wq

ocfs2_quota_wq is not depended upon during memory reclaim and, with
cmwq, there's no reason to use a dedicated workqueue.  Drop
ocfs2_quota_wq and use system_wq instead.  dqi_sync_work is already
sync canceled on quota disable and no further synchronization is
necessary.

This change makes ocfs2_quota_setup/shutdown() noops.  Both functions
removed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <joel.becker@oracle.com>