Flow creates a VRS client based on the `$wgVirtualRestConfig`. If the config sets `$wg...['modules']['restbase']`, then `RestbaseVirtualRestService` (disabled for Flow) could be used but if not, falls back to using `ParsoidVirtualRestService` (which is what is being used now) because using the `/restbase/` module drops data-parsoid mappings after transforming the wikitext -> html.
The main reason why Flow contacts Parsoid via RESTBase is to perform transformations to/from different formats (wikitext -> html, html->wikitext etc):
$url = '/restbase/local/v1/transform/' . $from . '/to/' . $to . '/' .
urlencode( $prefixedDbTitle );
Recently, we've moved this logic into MW core and based on various options (`$from`, `$to` etc) in the `Flow\Conversion\Utils::convert()` path, we can apply core's Parsoid logic to perform this transformation/conversion directly and cut ties with RESTBase.
NOTE: If parsoid isn't configured, we use the legacy MediaWiki parser which only transforms wikitext -> html.
== Acceptance Criteria ==
[ ] Put tests in place to capture different outputs after transformation is applied with RB configs in place
[ ] Remove usage of VirtualRestService and replace with core logic
[ ] Ensure the outputs returned by core code after transformation matches above (tests shouldn't break)
[ ] Review to make sure no VRS logic is still in extension
[ ] Test extension manually to make sure it still works as expected
[ ] Once deployed, monitor sites for any spikes or failures reported for a week or two.