Remove out-of-date comment.

The use of `Binder` was removed in the recent #123900, but the comment
wasn't removed at the same time.

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -3443,8 +3443,6 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         self.dcx().try_steal_replace_and_emit_err(self.tcx.def_span(def_id), StashKey::Cycle, err)
     }
 
-    // FIXME(@lcnr): This function could be changed to trait `TraitRef` directly
-    // instead of using a `Binder`.
     fn report_signature_mismatch_error(
         &self,
         obligation: &PredicateObligation<'tcx>,