Remove DisplayStructTraitsTest interface.

ui/display/mojo/BUILD.gn

 # Copyright 2017 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("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("interfaces") {
   sources = [
     "display.mojom",
     "display_constants.mojom",
     "display_layout.mojom",
     "display_mode.mojom",
     "display_snapshot_mojo.mojom",
     "gamma_ramp_rgb_entry.mojom",
     "native_display_delegate.mojom",
   ]
 
   public_deps = [
     "//mojo/common:common_custom_types",
     "//ui/gfx/geometry/mojo",
   ]
 }
-
-mojom("test_interfaces") {
-  sources = [
-    "display_struct_traits_test.mojom",
-  ]
-
-  public_deps = [
-    ":interfaces",
-  ]
-}