Cleanup: remove non-standard retain_unknown_fields from .proto files

google_apis/gcm/protocol/mcs.proto

 // Copyright 2013 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.
 //
 // MCS protocol for communication between Chrome client and Mobile Connection
 // Server .
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 
 package mcs_proto;
 
 /*
    Common fields/comments:
 
     stream_id: no longer sent by server, each side keeps a counter
     last_stream_id_received: sent only if a packet was received since last time
     a last_stream was sent
     status: new bitmask including the 'idle' as bit 0.
@@ skipping 257 matching lines @@
   // No last_streamid_received required.  This is included within an IqStanza,
   // which includes the last_stream_id_received.
 }
 
 /**
   Included in IQ sent after LoginResponse from server with ID 12.
 */
 message SelectiveAck {
   repeated string id = 1;
 }