Remove unnecessary line

Originally added on the basis we might care about tracking the number of
events lost from the queue, but that isn't necessary.

src/coniql/caplugin.py

@@ -396,7 +396,6 @@ def __callback(channel: Channel):
             while True:
                 try:
                     value.get_nowait()
-                    value.task_done()  # Not necessary as we never join on the queue
                 except asyncio.QueueEmpty:
                     break