useDeprecatedSynchronousErrorHandling not calling teardown if synchronously unsubscribed

[benlesh]

Found in google3 (of course, haha) cc @leggechr

import { Observable, config } from "rxjs";

config.useDeprecatedSynchronousErrorHandling = true;

let called = false;
const observable = new Observable(obs => () => (called = true));
const subscription = observable.subscribe();
subscription.unsubscribe();

console.log("called: ", called); // logs called: false

https://stackblitz.com/edit/rxjs-4stiyw?devtoolsheight=60