Remove obsolete poll-sync responders from PollingChannel and other impacted protocol-specific channels.
Checkintests Pass: Yes
This stuff was originally used by a PollingChannel to directly monitor subscribe/unsubscribe commands issued by higher-level components in order to drive a count of active subscriptions, and from that, whether or not the channel needed to issue poll requests.
However, DataService, and then Consumer ended up managing this directly because they had better insight into when they considered themselves subscribed and they notify the PollingChannel of their state changes directly (mx_internally). When this was added, the underlying poll-sync responder machinery didn't go away, and it generally doesn't cause problems except in the case of a client disconnect without a preceding explicit unsubscribe. In this case the higher level component moves to an unsubscribed state when it disconnects (and lets the PollingChannel know) but because no unsubscribe command is sent by a poll-sync responder, the extra count it added never decrements and the channel continues to poll incorrectly.