This was a holdover from the DG refactor.

When we migrated DeltaGenerator to start using mixins, there were some files existing for some of the protobuf marshalling, for example "image_proto.py". At the time, to keep the refactor small, we didn't rename those, but the newer mixin files were named eg "markdown.py".

This is the start of work to finally clean up all the _proto suffixes.

• data_frame_proto
• exception_proto
• iframe_proto
• image_proto
• media_proto