Remove content/public/plugin and chrome/plugin

content/public/plugin/BUILD.gn

-# 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.
-
-# See //content/BUILD.gn for how this works.
-group("plugin") {
-  if (is_component_build) {
-    public_deps = [
-      "//content",
-    ]
-  } else {
-    public_deps = [
-      ":plugin_sources",
-    ]
-  }
-}
-
-source_set("plugin_sources") {
-  # External code should depend on via ":plugin" above.
-  visibility = [ "//content/*" ]
-
-  sources = [
-    "content_plugin_client.h",
-  ]
-
-  deps = [
-    "//base",
-    "//content/public/common:common_sources",
-  ]
-}