Simplify rustc_ast::visit::Visitor::visit_poly_trait_ref.

It is passed an argument that is never used.

clippy_lints/src/unused_unit.rs

@@ -89,7 +89,7 @@ fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
         }
     }
 
-    fn check_poly_trait_ref(&mut self, cx: &EarlyContext<'_>, poly: &ast::PolyTraitRef, _: &ast::TraitBoundModifier) {
+    fn check_poly_trait_ref(&mut self, cx: &EarlyContext<'_>, poly: &ast::PolyTraitRef) {
         let segments = &poly.trait_ref.path.segments;
 
         if_chain! {