Remove media_galleries_handlers - not used.

chrome/common/extensions/api/_manifest_features.json

 // 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.
 
 // This features file defines manifest keys implemented under src/chrome.
 // See extensions/common/features/* to understand this file, in particular
 // feature.h, simple_feature.h, and base_feature_provider.h.
 
 // To add a new whitelisted ID, SHA-1 it and force it to uppercase. In Bash:
 //
@@ skipping 207 matching lines @@
     "extension_types": ["extension", "legacy_packaged_app"]
   },
   "key": {
     "channel": "stable",
     "extension_types": "all"
   },
   "manifest_version": {
     "channel": "stable",
     "extension_types": "all"
   },
-  "media_galleries_handlers": {
-    "channel": "dev",
-    "extension_types": ["platform_app"]
-  },
   "mime_types": {
     "channel": "stable",
     "extension_types": [ "extension", "legacy_packaged_app", "platform_app" ],
     "whitelist": [
       "oickdpebdnfbgkcaoklfcdhjniefkcji",  // browser_tests
       "gbkeegbaiigmenfmjfclcdgdpimamgkj",  // QuickOffice
       "ionpfmkccalenbmnddpbmocokhaknphg",  // QuickOffice Dev
       "bpmcpldpdmajfigpchkicefoigmkfalc",  // Quickoffice component extension
       "ehibbfinohgbchlgdbfpikodjaojhccn",  // Editor
       "mhjfbmdgcfjbbpaeojofohoefgiehjai"   // PDF
@@ skipping 147 matching lines @@
   },
   "url_handlers": {
     "channel": "stable",
     "extension_types": ["platform_app"]
   },
   "version": {
     "channel": "stable",
     "extension_types": "all"
   }
 }