Remove duplicated user_prefs::TestingPrefServiceSyncable

components/pref_registry/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 static_library("pref_registry") {
   sources = [
     "pref_registry_syncable.cc",
     "pref_registry_syncable.h",
   ]
 
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
     "//components/prefs",
   ]
 }
-
-static_library("test_support") {
-  testonly = true
-  sources = [
-    "testing_pref_service_syncable.cc",
-    "testing_pref_service_syncable.h",
-  ]
-
-  public_deps = [
-    ":pref_registry",
-  ]
-
-  deps = [
-    "//base",
-    "//components/prefs:test_support",
-  ]
-}