fix(core): remove @internal annotation from PLATFORM_CORE_PROVIDERS

This symbol is no longer reexported at the top level, so it's safe to not mark it as internal.

This fixes the offline compilation which got broken by this symbol not being present in the d.ts
files when the compiler tries to do a deep import.

Closes #8819

modules/@angular/core/src/application_ref.ts

@@ -418,9 +418,6 @@ export class ApplicationRef_ extends ApplicationRef {
   get componentTypes(): Type[] { return this._rootComponentTypes; }
 }
 
-/**
- * @internal
- */
 export const PLATFORM_CORE_PROVIDERS =
     /*@ts2dart_const*/[
       PlatformRef_,