Cleanup dummy query url processor.

Follow-up to #2755663: Add a solid slider and range widget base.

Part of the work we needed to do for #2755663: Add a solid slider and range widget base made us have to change the $separator from a contant to a protected variable. This means that we can refactor the class DummyQuery extends UrlProcessorPluginBase { into class DummyQuery extends QueryString { protected $separator = '||'; } and basically remove the entire implementation.

This doesn't change any of our runtime code but makes it easier to maintain the testsuite. Postponing this on that issue (that needs reviews + at least one test first).