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

mojo/public/tools/bindings/generators/cpp_templates/union_definition.tmpl

-{%- set class_name = union.name ~ "_Data" %}
-
-// static
-{{class_name}}* {{class_name}}::New(mojo::internal::Buffer* buf) {
-  return new (buf->Allocate(sizeof({{class_name}}))) {{class_name}}();
-}
-
-// static
-bool {{class_name}}::Validate(const void* data,
-                              mojo::internal::BoundsChecker* bounds_checker) {
-  // TODO(azani): Implement validation.
-  return true;
-}
-
-{{class_name}}::{{class_name}}() {
-}
-
-void {{class_name}}::EncodePointersAndHandles(
-    std::vector<mojo::Handle>* handles) {
-  // TODO(azani): Implement pointers and handles.
-}
-
-void {{class_name}}::DecodePointersAndHandles(
-    std::vector<mojo::Handle>* handles) {
-  // TODO(azani): Implement pointers and handles.
-}