Remove the subscriber when unsubscribe

[hez2010]

Fix the issue where unsubscribe doesn't remove the subscriber.
This can cause ObjectDisposedException later if the publisher publishes a new message after a subscriber unsubscribed and disconnected from the server.

[hez2010]

To reproduce the bug:

1. Run RedisInsight, goto the Sub/Pub page
2. Subscribe and publish some messages
3. Unsubscribe and then continue to publish some messages
4. See exception from the server

[badrishc]

LGTM, thanks!