refactor(testing): remove deprecated testing functions (#10832)

Remove TestComponentBuilder, addProviders, and withProviders. These
were deprecated in rc5 - see the changelog for update information.

Note - this does not actually remove the functions, but makes them
internal only. They will be removed from the codebase entirely
at a later time.

modules/@angular/core/testing/testing.ts

@@ -27,13 +27,6 @@ if (_global.beforeEach) {
   });
 }
 
-/**
- * Allows overriding default providers of the test injector,
- * which are defined in test_injector.js
- *
- * @deprecated Use `TestBed.configureTestingModule instead.
- */
-export function addProviders(providers: Array<any>): void {
-  if (!providers) return;
-  TestBed.configureTestingModule({providers: providers});
-}
+// TODO(juliemr): remove this, only used because we need to export something to have compilation
+// work.
+export var __core_private_testing_placeholder__ = '';