Remove unused issuable json endpoints

What does this MR do and why?

Removes the issues and MR JSON endpoints.

Also removes setting of instance variables that we do not use anywhere

I checked the logs and there are only 49 hits in the last 7 days: https://log.gprd.gitlab.net/app/r/s/uaou8. I think these are just users manually appending .json to the URL, with some of these made by me for testing.

It is very unlikely that anyone would depend on this endpoint because it returns data in a weird format. It's actually rendering the HTML view then returning it as JSON string.

How to set up and validate locally

1. Visit a project's issue / MR list and append .json to the URL
2. It should now return 404

MR acceptance checklist

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