FCPInterface.handleUnsubscribe(): Remove TODO about cleaning up the
ClientTrackerDaemon

- With fred plugin-fcp-rewrite, the client objects are managed
automatically by fred. The ClientTrackerDaemon only still exists for
unit tests, and they won't need cleanup.
FCPInteface: Rename "fcpID" variables to "clientID"

- Those are all IDs of a FCPPlugin*Client*
FCPInterface.handleUnsubscribe(): Adapt to
plugin-fcp-rewrite

- Return a FCPPluginMessage instead of SFS since
sendUnsubscribedMessage() will want to pass the return value of
handleUnsubscribe() into FCPPluginClient.send()