Remove duplicate netty dependency

Cassandra includes a later version of Netty (3.8.0) which has the same package structure (org.jboss.netty..), so I'm not sure you need to explicitly include an earlier version here.

I have had this cause problems where classpath-ordering was causing the wrong version of the netty jar to be used, leading me to explicitly exclude 3.2.9.Final from Cassandra-Unit. Thought a pull request might help.