Remove hard-coded use of main class in ivory script

This was broken in 0fa22ddbb587be1f350cd34e895d859c2f0684c7

http://www.roman10.net/specify-the-main-class-in-a-jar-file-for-hadoop/

> If a jar file contains a main class specified in its manifest file,
> hadoop will take the main class even if the command specify another main class.

/cc @markhibberd