Correct a comment.

I tried simplifying `RegionCtxt`, which led me to finding that the
fields are printed in `sccs_info`.

Author: nnethercote

compiler/rustc_borrowck/src/renumber.rs

@@ -26,9 +26,7 @@ pub fn renumber_mir<'tcx>(
     renumberer.visit_body(body);
 }
 
-// FIXME(@lcnr): A lot of these variants overlap and it seems like
-// this type is only used to decide which region should be used
-// as representative. This should be cleaned up.
+// The fields are used only for debugging output in `sccs_info`.
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
 pub(crate) enum RegionCtxt {
     Location(Location),