Problem/Motivation
This is a follow-up to #2894270: Users unable to add an extension to the file upload field
Okay, well, so there isn't test coverage for ensuring the thing can't be deleted. So I'd recommend a separate issue, "Add test coverage ensuring that the media source field cannot be removed from a media type (and remove unneeded media_entity_operations_alter())"
Proposed resolution
- Write tests
- Remove media_entity_operations_alter implementation