Get rid of BLI_task_pool_stop().

Comments said that function was supposed to 'stop worker threads', but
it absolutely did not do anything like that, was merely wiping out TODO
queue of tasks from given pool (kind of subset of what
`BLI_task_pool_cancel()` does).

Misleading, and currently useless, we can always add it back if we need
it some day, but for now we try to simplify that area.

source/blender/blenlib/intern/task.c

@@ -708,13 +708,6 @@ void BLI_task_pool_cancel(TaskPool *pool)
 	pool->do_cancel = false;
 }
 
-void BLI_task_pool_stop(TaskPool *pool)
-{
-	task_scheduler_clear(pool->scheduler, pool);
-
-	BLI_assert(pool->num == 0);
-}
-
 bool BLI_task_pool_canceled(TaskPool *pool)
 {
 	return pool->do_cancel;