Remove TypeSuper{Foldable,Visitable} impls for Region.

These traits exist so that folders/visitors can recurse into types of
interest: binders, types, regions, predicates, and consts. But `Region`
is non-recursive and cannot contain other types of interest, so its
methods in these traits are trivial.

This commit inlines and removes those trivial methods.

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -450,7 +450,7 @@ fn is_impossible_method(tcx: TyCtxt<'_>, (impl_def_id, trait_item_def_id): (DefI
             {
                 return ControlFlow::Break(());
             }
-            r.super_visit_with(self)
+            ControlFlow::Continue(())
         }
         fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
             if let ty::ConstKind::Param(param) = ct.kind()