Only use mixed formulation method

It's annoying to have to juggle between the mixed formulation class (`Disctontinuous`) and the standard one. We could just use mixed-formulation all the time

No file or test is named. Start by locating the standard and mixed-formulation classes, including `Disctontinuous`, and compare where each is selected and used. Define the scope with maintainers, then verify that the standard path is removed or unified without breaking existing hydrogen, tritium, or heat-transfer models.

Written by the indexing model from the issue text.