Remove //gpu and other unused code

Oneline diffstat: 489 files changed, 4 insertions(+), 186980
deletions(-)

This deletes all of //gpu (except for one export header) and
various bits of //mojo/{gpu,gles2} that are
not used. SkyShell on Android/iOS/etc bind to the system's GL
driver via //ui/gl. sky_viewer.mojo
binds to the Mojo C entry points.

gpu/BUILD.gn

@@ -1,15 +1 @@
-# 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.
 
-config("gpu_implementation") {
-  defines = [ "GPU_IMPLEMENTATION" ]
-}
-
-component("gpu") {
-  public_deps = [
-    "//gpu/command_buffer/client",
-    "//gpu/command_buffer/common",
-    "//gpu/config",
-  ]
-}