Re-enable ConstArgKind::Path lowering by default

...and remove the `const_arg_path` feature gate as a result. It was only
a stopgap measure to fix the regression that the new lowering introduced
(which should now be fixed by this PR).

compiler/rustc_ast_lowering/src/expr.rs

@@ -387,7 +387,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let node_id = self.next_node_id();
 
                 // HACK(min_generic_const_args): see lower_anon_const
-                if !self.tcx.features().const_arg_path || !arg.is_potential_trivial_const_arg() {
+                if !arg.is_potential_trivial_const_arg() {
                     // Add a definition for the in-band const def.
                     self.create_def(parent_def_id, node_id, kw::Empty, DefKind::AnonConst, f.span);
                 }