Remove stale identity_test_utils.h include

Forgot to do this (again) in the CL converting away from these utils.

TBR=droger@chromium.org

Bug: 896180
Change-Id: I3845417f5884d5adfb048d919931a6e021221e6f
Reviewed-on: https://chromium-review.googlesource.com/c/1288869Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600781}

chrome/browser/profiles/gaia_info_update_service_unittest.cc

@@ -29,7 +29,6 @@
 #include "components/signin/core/browser/account_tracker_service.h"
 #include "components/signin/core/browser/signin_pref_names.h"
 #include "components/sync_preferences/pref_service_syncable.h"
-#include "services/identity/public/cpp/identity_test_utils.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "ui/gfx/image/image.h"
 #include "ui/gfx/image/image_unittest_util.h"