Bug 1642493: Clean up remaining references to ENABLE_NEW_REGEXP and old irregexp r=mgaudet

I kept a running list of code that would be dead once we removed the old engine. This is what was on that list.

Note: The regexp_parse tests used a testing function that parsed a regexp and returned an AST to compare against the expected AST. They are being deleted because the tests are fragile in the face of upstream changes, not particularly useful, and a lot of work to convert.

Depends on D77727

Differential Revision: https://phabricator.services.mozilla.com/D77728