After the transition from Parsoid-JS to Parsoid-PHP has been done and deployed, we need to clear up the code supporting the transition from RESTBase.
The code in question was implemented in a single PR. This code should mostly be reverted with a little bits left intact (adding the host header to parsoid request). Unfortunately, the transition included a lot of followup changes, so most likely automatic git-based revert will not work.
Additionally, as soon as we loose the capability to access JS Parsoid, we'd have problems with tests that rely on production enwiki (see T231933), so that should be fixed first.