Revert of Add gclient hook to download prebuilt mojo_shell

mojo/public/tools/BUILD.gn

Index: mojo/public/tools/BUILD.gn
diff --git a/mojo/public/tools/BUILD.gn b/mojo/public/tools/BUILD.gn
deleted file mode 100644
index 6405c544be24321d5dd252e07a6c32ed9e7c0654..0000000000000000000000000000000000000000
--- a/mojo/public/tools/BUILD.gn
+++ /dev/null
@@ -1,16 +0,0 @@
-# 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.
-
-import("//mojo/public/mojo.gni")
-
-if (use_prebuilt_mojo_shell) {
-  copy("copy_mojo_shell") {
-    filename = "mojo_shell"
-    if (is_win) {
-      filename += ".exe"
-    }
-    sources = [ "prebuilt/$filename" ]
-    outputs = [ "$root_out_dir/$filename" ]
-  }
-}