Turn off Javadoc errors for Netty Utils

We don't Javadoc to tell us about errors in Netty Utils since this project has just been copied from Netty. The errors I saw were about missing references back to other code in Netty.

This error shows up when building using Java 8 and Scala 2.11. Under Java 7 the javadoc tool is more forgiving. Javadoc doesn't seem to run at all when building with Scala 2.10.