Remove GN GYP generator.

tools/gn/secondary/sdch/BUILD.gn

 # Copyright (c) 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.
 
 config("sdch_config") {
   include_dirs = [ "open-vcdiff/src" ]
 }
 
 static_library("sdch") {
-  external = true
   sources = [
     "open-vcdiff/src/addrcache.cc",
     "open-vcdiff/src/blockhash.cc",
     "open-vcdiff/src/blockhash.h",
     "open-vcdiff/src/checksum.h",
     "open-vcdiff/src/codetable.cc",
     "open-vcdiff/src/codetable.h",
     "open-vcdiff/src/compile_assert.h",
     "open-vcdiff/src/decodetable.cc",
     "open-vcdiff/src/decodetable.h",
@@ skipping 26 matching lines @@
   } else if (is_ios) {
     include_dirs = [ "ios" ]
   } else if (is_mac) {
     include_dirs = [ "mac" ]
   } else if (is_win) {
     include_dirs = [ "win" ]
   }
 
   deps = [ "//third_party/zlib" ]
 }