Remove enhanced bookmarks sync experiment

sync/protocol/experiments_specifics.proto

 // Copyright (c) 2012 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.
 //
 // Sync protocol datatype extension for experimental feature flags.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 option retain_unknown_fields = true;
@@ skipping 52 matching lines @@
 // Contains one flag or set of related flags.  Each node of the experiments type
 // will have a unique_client_tag identifying which flags it contains.  By
 // convention, the tag name should match the sub-message name.
 message ExperimentsSpecifics {
   optional KeystoreEncryptionFlags keystore_encryption = 1;
   optional HistoryDeleteDirectives history_delete_directives = 2;
   optional AutofillCullingFlags autofill_culling = 3;
   optional FaviconSyncFlags favicon_sync = 4;
   optional PreCommitUpdateAvoidanceFlags pre_commit_update_avoidance = 5;
   optional GcmChannelFlags gcm_channel = 6;
-  optional EnhancedBookmarksFlags enhanced_bookmarks = 7;
+  // No longer used as of M43.
+  optional EnhancedBookmarksFlags obsolete_enhanced_bookmarks = 7;
   optional GcmInvalidationsFlags gcm_invalidations = 8;
   optional WalletSyncFlags wallet_sync = 9;
 }