Remove errant on_prompt handler

The orchestration code included functionality to track when execution
had completed which is not useful outside of orchestration. As the other
references to completion had been removed, this code was causing
exceptions during execution.

workflowcheckpointing.py

@@ -325,13 +325,6 @@ async def fetch_remote_files(remote_files, uid=None):
     if len(fetches) > 0:
         await asyncio.gather(*fetches)
 
-completion_futures = {}
-def add_future(json_data):
-    index = max(completion_futures.keys())
-    json_data['extra_data']['completion_future'] = index
-    return json_data
-server.PromptServer.instance.add_on_prompt_handler(add_future)
-
 prompt_route = next(filter(lambda x: x.path  == '/prompt' and x.method == 'POST',
                            server.PromptServer.instance.routes))
 original_post_prompt = prompt_route.handler