Avoid double-interning some BoundVariableKinds.

This function has this line twice:
```
let bound_vars = tcx.intern_bound_variable_kinds(&bound_vars);
```
The second occurrence is effectively a no-op, because the first
occurrence interned any that needed it.

Author: nnethercote

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -566,8 +566,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         });
                         let bound_vars = tcx.intern_bound_variable_kinds(&bound_vars);
                         let assoc_ty_substs = tcx.intern_substs(&substs);
-
-                        let bound_vars = tcx.intern_bound_variable_kinds(&bound_vars);
                         let bound =
                             bound.map_bound(|b| b.kind().skip_binder()).subst(tcx, assoc_ty_substs);
                         tcx.mk_predicate(ty::Binder::bind_with_vars(bound, bound_vars))