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_span/src/symbol.rs

@@ -599,7 +599,6 @@ symbols! {
         conservative_impl_trait,
         console,
         const_allocate,
-        const_arg_path,
         const_async_blocks,
         const_closures,
         const_compare_raw_pointers,