7908b8d7be2a2992c57cd549054eda7ce46e4b44 ("m-lua-scripting: allow converting GValue holding NULL objects to Lua") accidentally added a second refcount. As a result, the objects are never freeded. Remove the second refcount to fix this.
It's unlikely that wplua will ever be useful outside the context of module-lua-scripting, so let's move it to keep all the code in one place