Remove an unused instance of FakeGaiaCookieManagerService.

Tests which call GetIdentityTestEnvironmentFactories() don't set any
fake cookie responses, so they don't need to create a FakeGCMS.

This CL is part of eliminating FakeGCMS entirely:
https://docs.google.com/document/d/1t0ZtuV7h-znzdItFgBW0aKPscAwWXIBuNZnNlEGgi7g/edit


Bug: 907782
Change-Id: Ic42dcd215ecf9ca162e96cfa37d3424ead703ea7
Reviewed-on: https://chromium-review.googlesource.com/c/1445909
Commit-Queue: Lowell Manners <lowell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627967}

ios/chrome/browser/signin/identity_test_environment_chrome_browser_state_adaptor.cc

@@ -35,9 +35,7 @@ std::unique_ptr<KeyedService> BuildFakeSigninManager(
 }
 
 TestChromeBrowserState::TestingFactories GetIdentityTestEnvironmentFactories() {
-  return {{ios::GaiaCookieManagerServiceFactory::GetInstance(),
-           base::BindRepeating(&BuildFakeGaiaCookieManagerService)},
-          {ProfileOAuth2TokenServiceFactory::GetInstance(),
+  return {{ProfileOAuth2TokenServiceFactory::GetInstance(),
            base::BindRepeating(&BuildFakeOAuth2TokenService)},
           {ios::SigninManagerFactory::GetInstance(),
            base::BindRepeating(&BuildFakeSigninManager)}};