libutil: remove long-dead create_coro_gc_hook

Change-Id: Ia37c6a5401dbe6453bfdfa733da5237d2c2dc819

lix/libutil/serialise.hh

@@ -598,10 +598,4 @@ struct FramedSink : nix::BufferedSink
     };
 };
 
-/* Disabling GC when entering a coroutine (without the boehm patch).
-   mutable to avoid boehm gc dependency in libutil.
- */
-extern std::shared_ptr<void> (*create_coro_gc_hook)();
-
-
 }