Remove global Observable<T>["@@observable"] method declaration.

[benjamn]

Should fix #7839.

I find it surprising that setting Observable.prototype['@@observable'] is necessary in addition to setting Observable.prototype[Symbol.observable] (which zen-observable already does), but if you comment out

  prototype[fakeObsSymbol] = function () { return this; };

and run our test suite, you will see failures:

 FAIL  core/__tests__/QueryManager/index.ts (20.796 s)
  ● QueryManager › supports interoperability with other Observable implementations like RxJS

    TypeError: You provided an invalid object where a stream was expected. You can provide an Observable, Promise, Array, or Iterable.

      577 |     });
      578 | 
    > 579 |     const observable = from(handle as any);
          |                        ^
      580 | 
      581 |     observable.pipe(map(result => assign({ fromRx: true }, result))).subscribe({
      582 |       next: wrap(reject, newResult => {

      at Object.<anonymous>.exports.subscribeTo (../node_modules/rxjs/src/internal/util/subscribeTo.ts:27:11)
      at Object.from (../node_modules/rxjs/src/internal/observable/from.ts:114:30)
      at Object.<anonymous> (core/__tests__/QueryManager/index.ts:579:24)
      at utilities/testing/itAsync.ts:11:37
      at Object.<anonymous> (utilities/testing/itAsync.ts:10:12)

[brainkim]

👍 Interesting that the issue requires runtime changes, and wondering why the TypeError wasn’t thrown before or caught by the test suite.

[benjamn]

@brainkim This PR would solve #7839 if I merely deleted the declare global { ... } part, but I decided to make the runtime part slightly more defensive while I was at it.