[Streams] Remove stream alert if stream not found (#4939)

* Remove stream if not found

* Address Jack's review.

redbot/cogs/streams/streams.py

@@ -778,6 +778,9 @@ class Streams(commands.Cog):
 
                     stream.messages.clear()
                     await self.save_streams()
+                except StreamNotFound:
+                    to_remove.append(stream)
+                    continue
                 else:
                     if stream.messages:
                         continue