Rollup merge of #99638 - GuillaumeGomez:rm-clean-trait, r=notriddle

Remove Clean trait implementation for hir::Ty and middle::Ty

While going through the code, I realized that the "remove the Clean trait" effort which was started a while ago was never finished.

The idea behind it was to make it much simpler to go through the different clean steps (which is definitely not easy when you just have `something.clean(cx)`).

I'm planning to finish this effort.

cc ``@camelid``
r? ``@notriddle``