Remove fold_const override for ReverseMapper

Co-Authored-By: Gabriel Smith <yodaldevoid@users.noreply.github.com>

src/librustc/infer/opaque_types/mod.rs

@@ -659,10 +659,6 @@ impl<'cx, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ReverseMapper<'cx, 'gcx, 'tcx>
             _ => ty.super_fold_with(self),
         }
     }
-
-    fn fold_const(&mut self, ct: &'tcx ty::LazyConst<'tcx>) -> &'tcx ty::LazyConst<'tcx> {
-        ct // FIXME(const_generics)
-    }
 }
 
 struct Instantiator<'a, 'gcx: 'tcx, 'tcx: 'a> {