Remove FOR UPDATE in Queue cancel and complete

We have no need for them anymore and it should reduce lock contention a bit

Fixes #776

data/queue.py

@@ -276,29 +276,11 @@ class WorkQueue(object):
     """ Attempts to cancel the queue item with the given ID from the queue. Returns true on success
         and false if the queue item could not be canceled.
     """
-
-    with self._transaction_factory(db):
-      # Load the build queue item for update.
-      try:
-        queue_item = db_for_update(QueueItem.select()
-                                   .where(QueueItem.id == item_id)).get()
-      except QueueItem.DoesNotExist:
-        return False
-
-      # Delete the queue item.
-      queue_item.delete_instance(recursive=True)
-      return True
+    count_removed = QueueItem.delete().where(QueueItem.id == item_id).execute()
+    return count_removed > 0
 
   def complete(self, completed_item):
-    with self._transaction_factory(db):
-      try:
-        completed_item_obj = self._item_by_id_for_update(completed_item.id)
-      except QueueItem.DoesNotExist:
-        self._currently_processing = False
-        return
-
-      completed_item_obj.delete_instance(recursive=True)
-      self._currently_processing = False
+    self._currently_processing = not self.cancel(completed_item.id)
 
   def incomplete(self, incomplete_item, retry_after=300, restore_retry=False):
     with self._transaction_factory(db):