Remove hard-coded use of main class in ivory script

This was broken in 0fa22ddbb587be1f350cd34e895d859c2f0684c7
because setting mainClass in assembly is not enough.
It's better to specify this directly in the build anyway.

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.