Silence `api_json_content_type` being not used [RUN ALL RSPEC]

What does this MR do?

This is done to silence the #271554 (closed).

I tried to have a proper coverage for this FF, but it is hard:

• !42229 (comment 434994027)
• #271556

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content