Drop the unreachable branch in the live socket loop

`asyncio.wait(..., FIRST_COMPLETED)` returns with at least one of the two
tasks done, so `receiver` missing from `done` already means `sender` is in
it. The `elif sender not in done: continue` could never fire.

backend/fluksio/api/routes/flows.py

@@ -1076,8 +1076,8 @@ async def flow_events(websocket: WebSocket, token: str = "") -> None:
                     receiver = asyncio.create_task(websocket.receive_text())
                     if sender not in done:
                         continue
-                elif sender not in done:
-                    continue
+                # `sender` is done from here: `FIRST_COMPLETED` returns with at
+                # least one of the two, so `receiver` missing means this one is.
 
                 # Everything the bus has right now, not just the one event
                 # that woke this: a cascade puts a dozen in at once, and one