Convert CallHandler to typescript

and remove the old conference call stuff while we're at it: enough
time should have passed since those mistakes that we can move on.
The old conference call rooms will still appear for anyone whose
account dates back to that time, but they've presumably been appearing
in any other matrix client they used too.

src/components/structures/MatrixChat.tsx

@@ -148,7 +148,6 @@ interface IRoomInfo {
 interface IProps { // TODO type things better
     config: Record<string, any>;
     serverConfig?: ValidatedServerConfig;
-    ConferenceHandler?: any;
     onNewScreen: (screen: string, replaceLast: boolean) => void;
     enableGuest?: boolean;
     // the queryParams extracted from the [real] query-string of the URI