Cleanup GN BUILD files, gni's and imports.

mandoline/app/BUILD.gn

 # Copyright 2015 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("//third_party/mojo/src/mojo/public/mojo.gni")
-
 group("app") {
   deps = [
     "//mandoline/ui",
   ]
 
   if (is_android) {
     deps += [ "//mandoline/app/android" ]
   } else {
     deps += [ "//mandoline/app/desktop" ]
   }
 
   data_deps = [
     "//mandoline/ui/browser",
     "//mandoline/services/core_services",
     "//mojo/runner:mojo_runner",
   ]
 }