Revert "Generate some of the C++ bindings for mojom tagged unions."

mojo/public/cpp/bindings/tests/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.
 
 import("../../../mojo_sdk.gni")
 
 mojo_sdk_source_set("tests") {
   testonly = true
 
   sources = [
     "array_unittest.cc",
     "bounds_checker_unittest.cc",
     "buffer_unittest.cc",
     "connector_unittest.cc",
     "container_test_util.cc",
     "equals_unittest.cc",
     "handle_passing_unittest.cc",
     "interface_ptr_unittest.cc",
     "map_unittest.cc",
     "request_response_unittest.cc",
     "router_unittest.cc",
     "sample_service_unittest.cc",
     "serialization_warning_unittest.cc",
     "string_unittest.cc",
     "struct_unittest.cc",
     "type_conversion_unittest.cc",
-    "union_unittest.cc",
     "validation_unittest.cc",
   ]
 
   deps = [
     ":mojo_public_bindings_test_utils",
     "//testing/gtest",
   ]
 
   mojo_sdk_deps = [
     "mojo/public/cpp/bindings",
     "mojo/public/cpp/environment:standalone",
     "mojo/public/cpp/system",
     "mojo/public/cpp/test_support:test_utils",
     "mojo/public/cpp/utility",
     "mojo/public/interfaces/bindings/tests:test_interfaces",
   ]
 }
 
 mojo_sdk_source_set("mojo_public_bindings_test_utils") {
   sources = [
     "validation_test_input_parser.cc",
     "validation_test_input_parser.h",
   ]
 
   mojo_sdk_deps = [ "mojo/public/c/system" ]
 }