Mojo cpp bindings: remove redundant validation in Decode*().

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

 {%- macro validate(struct, class_name) %}
   if (!data)
     return true;
 
   if (!ValidateStructHeader(
           data, sizeof({{class_name}}),
           {{struct.packed.packed_fields|length}}, bounds_checker)) {
     return false;
   }
 
@@ skipping 60 matching lines @@
 {%-   for pf in struct.packed.packed_fields if pf.field.kind|is_object_kind -%}
 mojo::internal::Encode(&{{pf.field.name}}, handles);
 {%    endfor %}
 {%-   for pf in struct.packed.packed_fields if pf.field.kind|is_handle_kind -%}
 mojo::internal::EncodeHandle(&{{pf.field.name}}, handles);
 {%    endfor %}
 {%- endmacro -%}
 
 {%- macro decodes(struct) -%}
 {%-   for pf in struct.packed.packed_fields if pf.field.kind|is_object_kind -%}
-if (!mojo::internal::Decode(&{{pf.field.name}}, message))
-  return false;
+mojo::internal::Decode(&{{pf.field.name}}, handles);
 {%    endfor %}
 {%-   for pf in struct.packed.packed_fields if pf.field.kind|is_handle_kind -%}
-if (!mojo::internal::DecodeHandle(&{{pf.field.name}},
-                                  message->mutable_handles()))
-  return false;
+mojo::internal::DecodeHandle(&{{pf.field.name}}, handles);
 {%    endfor %}
 {%- endmacro -%}