Remove some unnecessary skip_binder calls.

`is_fn_trait_compatible` is defined on both `FnSig` and `Binder<FnSig>`.

Author: nnethercote

compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs

@@ -461,15 +461,15 @@ pub(in crate::solve) fn extract_tupled_inputs_and_output_from_async_callable<I:
 
         ty::FnDef(def_id, _) => {
             let sig = self_ty.fn_sig(cx);
-            if sig.skip_binder().is_fn_trait_compatible() && !cx.has_target_features(def_id) {
+            if sig.is_fn_trait_compatible() && !cx.has_target_features(def_id) {
                 fn_item_to_async_callable(cx, sig)
             } else {
                 Err(NoSolution)
             }
         }
         ty::FnPtr(..) => {
             let sig = self_ty.fn_sig(cx);
-            if sig.skip_binder().is_fn_trait_compatible() {
+            if sig.is_fn_trait_compatible() {
                 fn_item_to_async_callable(cx, sig)
             } else {
                 Err(NoSolution)