Mus: Delete unused Mandoline Surfaces code

components/mus/public/cpp/surfaces/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.
 
 component("surfaces") {
   output_name = "mojo_surfaces_lib"
 
   sources = [
-    "custom_surface_converter.h",
     "mojo_surfaces_export.h",
     "surfaces_type_converters.cc",
     "surfaces_type_converters.h",
     "surfaces_utils.cc",
     "surfaces_utils.h",
   ]
 
   defines = [ "MOJO_SURFACES_IMPLEMENTATION" ]
 
   public_deps = [
     "//cc/ipc:interfaces",
     "//components/mus/public/interfaces",
     "//gpu",
     "//skia",
   ]
 
   deps = [
     "//base",
     "//cc",
     "//cc/surfaces",
     "//cc/surfaces:surface_id",
     "//ui/gfx",
     "//ui/gfx/geometry",
   ]
 }