refactor: remove unused promise completer (#5522)

* The PromiseCompleter is not used anywhere and in reality everbody can implement such a thing on his own. Also since there is the best practice to resolve/reject the promise while being inside of the anonymous function.

src/lib/core/async/promise-completer.ts

@@ -1,21 +0,0 @@
-/**
- * @license
- * Copyright Google Inc. All Rights Reserved.
- *
- * Use of this source code is governed by an MIT-style license that can be
- * found in the LICENSE file at https://angular.io/license
- */
-
-/** @docs-private */
-export class PromiseCompleter<R> {
-  promise: Promise<R>;
-  resolve: (value?: R | PromiseLike<R>) => void;
-  reject: (error?: any, stackTrace?: string) => void;
-
-  constructor() {
-    this.promise = new Promise((res, rej) => {
-      this.resolve = res;
-      this.reject = rej;
-    });
-  }
-}