Please keep in mind that you should do this as a last resort for plugin compatibility or CPU usage problems. By disabling Netty you are disabling the ultra efficient non-blocking networking I/O from Spigot which does many, many sexy things for your server.
It's more complicated than a simple bukkit.yml because the networking engine is started before the configuration is loaded, therefore you will need to edit your server's startup script to disable Netty.
Just add this JVM flag to your server startup script and Netty will be disabled.
Why can I only do this on builds 1138 and below?
While previously Spigot had rewritten the Minecraft server networking engine to use Netty instead of the standard vanilla networking engine, with Minecraft version 1.7 and above, Minecraft now utilizes Netty by default. Due to Minecraft's tight coupling with Netty, removing it would render your server unable to start.