workqueue: remove PREPARE_[DELAYED_]WORK()

Peter Hurley noticed that since a2c1c57b ("workqueue: consider
work function when searching for busy work items"), a work item which
gets assigned a different work function would break out of the
non-reentrancy guarantee as workqueue would consider it a different
work item.

This is fragile and extremely subtle.  PREPARE_[DELAYED_]WORK() have
never been used widely and its semantics has always been somewhat
iffy.  If the work item is known not to be on queue when
PREPARE_WORK() is called, there's no difference from using
INIT_WORK().  If the work item may be queued at the time of
PREPARE_WORK(), we can't really tell whether the old or new function
will be executed the next time.

We really don't want this level of subtlety in workqueue interface for
such marginal use cases.  The previous patches converted all existing
users away from PREPARE_[DELAYED_]WORK().  Let's remove them.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Peter Hurley <peter@hurleysoftware.com>
Link: http://lkml.kernel.org/g/1392493119-9277-1-git-send-email-peter@hurleysoftware.com