refactor(core): remove unused field in ApplicationRef class

This commit removes an unused field in the `ApplicationRef` class. Most likely the usage of the field was removed earlier.

Author: AndrewKushnir

packages/core/src/application/application_ref.ts

@@ -300,8 +300,6 @@ export function optionsReducer<T extends Object>(dst: T, objs: T | T[]): T {
  */
 @Injectable({providedIn: 'root'})
 export class ApplicationRef {
-  /** @internal */
-  private _bootstrapListeners: ((compRef: ComponentRef<any>) => void)[] = [];
   /** @internal */
   _runningTick: boolean = false;
   private _destroyed = false;
@@ -792,7 +790,7 @@ export class ApplicationRef {
           '`multi: true` provider.',
       );
     }
-    [...this._bootstrapListeners, ...listeners].forEach((listener) => listener(componentRef));
+    listeners.forEach((listener) => listener(componentRef));
   }
 
   /** @internal */
@@ -811,7 +809,6 @@ export class ApplicationRef {
 
       // Release all references.
       this._views = [];
-      this._bootstrapListeners = [];
       this._destroyListeners = [];
     }
   }