Revert of Add gclient hook to download prebuilt mojo_shell

mojo/public/mojo.gni

Index: mojo/public/mojo.gni
diff --git a/mojo/public/mojo.gni b/mojo/public/mojo.gni
deleted file mode 100644
index 5ff29890a3402a05fe434bf866c2a4ef7ee9e865..0000000000000000000000000000000000000000
--- a/mojo/public/mojo.gni
+++ /dev/null
@@ -1,9 +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.
-
-declare_args() {
-  # Whether to use a prebuilt mojo_shell binary instead of one built from
-  # source.
-  use_prebuilt_mojo_shell = false
-}