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

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

-{%- set class_name = union.name ~ "_Data" -%}
-{%- set enum_name = union.name ~ "_Tag" -%}
-{%- set union_name = union.name ~ "_Union" -%}
-
-class {{class_name}} {
- public:
-  static {{class_name}}* New(mojo::internal::Buffer* buf);
-
-  static bool Validate(const void* data,
-                       mojo::internal::BoundsChecker* bounds_checker);
-
-  enum class {{enum_name}} : uint64_t {
-{%  for field in union.fields %}
-    {{field.name|upper}},
-{%- endfor %}
-  };
-
-  union MOJO_ALIGNAS(8) {{union_name}} {
-{%  for field in union.fields %}
-    {{field.kind|cpp_field_type}} {{field.name}};
-{%- endfor %}
-  };
-
-  {{enum_name}} tag;
-  {{union_name}} data;
-
-  void EncodePointersAndHandles(std::vector<mojo::Handle>* handles);
-  void DecodePointersAndHandles(std::vector<mojo::Handle>* handles);
- 
- private:
-  {{class_name}}();
-  ~{{class_name}}() = delete;
-};
-static_assert(sizeof({{class_name}}) == 16,
-              "Bad sizeof({{class_name}})");