workqueue: remove in_workqueue_context()

Commit a25909a4 (lockdep: Add an in_workqueue_context() lockdep-based
test function) added in_workqueue_context() but there hasn't been any
in-kernel user and the lockdep annotation in workqueue is scheduled to
change.  Remove the unused function.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/workqueue.h

@@ -430,8 +430,4 @@ extern bool freeze_workqueues_busy(void);
 extern void thaw_workqueues(void);
 #endif /* CONFIG_FREEZER */
 
-#ifdef CONFIG_LOCKDEP
-int in_workqueue_context(struct workqueue_struct *wq);
-#endif
-
 #endif