Fixing issue with duplicate live query events after unsubscribe

[dbouianov]

Fixing issue with duplicate live query events after unsubscribe by removing subscriptions in unsubscribe method.

Repro scenario:

private ParseLiveQueryClient _client;
private ParseQuery _query;
private Subscription _sub;

public void InitLiveQuery() {
var cfg = new ParseClient.Configuration(...);
ParseClient.Initialize(cfg);
_client = new ParseLiveQueryClient(new Uri(serverUrl));
}

public void StartLiveQuery(string className, string objectId) {
_client.Reconnect();
_query = new ParseQuery(className);
if (!string.IsNullOrEmpty(objectId)) _query = _query.WhereEqualTo("objectId", objectId);
_sub = _client.Subscribe(_query);
_sub.HandleEvents(HandleLiveQueryEvent);
_sub.HandleError(HandleLiveQueryError);
}

public void StopLiveQuery() {
_client.Unsubscribe(_query, _sub);
_client.Disconnect();
_query = null;
_sub = null;
}

InitLiveQuery();
//not receiving live query update callbacks
StartLiveQuery();
//receiving live query update callbacks
StopLiveQuery();
//still receiving live query update callbacks
StartLiveQuery();
//receiving duplicate live query update callbacks

Root cause: subscriptions not being removed from subscription dictionary on unsubscribe.
Fix: removing subscriptions from subscription dictionary in unsubscribe method