Clean up old legacy preference migration logic.

components/user_prefs/tracked/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("user_prefs_tracked") {
   sources = [
     "device_id.h",
     "device_id_mac.cc",
     "device_id_stub.cc",
     "device_id_win.cc",
     "dictionary_hash_store_contents.cc",
     "dictionary_hash_store_contents.h",
     "hash_store_contents.h",
     "interceptable_pref_filter.cc",
     "interceptable_pref_filter.h",
     "pref_hash_calculator.cc",
     "pref_hash_calculator.h",
     "pref_hash_filter.cc",
     "pref_hash_filter.h",
     "pref_hash_store.h",
     "pref_hash_store_impl.cc",
     "pref_hash_store_impl.h",
     "pref_hash_store_transaction.h",
     "pref_names.cc",
     "pref_names.h",
-    "pref_service_hash_store_contents.cc",
-    "pref_service_hash_store_contents.h",
     "segregated_pref_store.cc",
     "segregated_pref_store.h",
     "tracked_atomic_preference.cc",
     "tracked_atomic_preference.h",
     "tracked_preference.h",
     "tracked_preference_helper.cc",
     "tracked_preference_helper.h",
     "tracked_preference_histogram_names.cc",
     "tracked_preference_histogram_names.h",
     "tracked_preference_validation_delegate.h",
@@ skipping 31 matching lines @@
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "device_id_unittest.cc",
     "pref_hash_calculator_unittest.cc",
     "pref_hash_filter_unittest.cc",
     "pref_hash_store_impl_unittest.cc",
-    "pref_service_hash_store_contents_unittest.cc",
     "segregated_pref_store_unittest.cc",
     "tracked_preferences_migration_unittest.cc",
   ]
 
   deps = [
     ":user_prefs_tracked",
     ":user_prefs_tracked_test_support",
     "//base:base",
     "//components/prefs:test_support",
     "//testing/gtest",
   ]
 }