remove outdated fixme comment

The `TraitPredicate` no longer has constness as we have desugared it to work with the type system through const generics instead.

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -102,7 +102,6 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
             if trait_clause.def_id() == goal.predicate.def_id()
                 && trait_clause.polarity() == goal.predicate.polarity
             {
-                // FIXME: Constness
                 ecx.probe_misc_candidate("assumption").enter(|ecx| {
                     let assumption_trait_pred = ecx.instantiate_binder_with_infer(trait_clause);
                     ecx.eq(