This jar requires org.mortbay.j2ee.jar to be deployed. Which is in jbossweb.sar which is actually Jetty.
I beleive you can remove that jar file safely since you don't use Jetty but I don't really know what it is for.
Ask it in jboss user list it's quite responsive to Jetty problems.
For what it's worth, I figured out a fix for this.
Download JBoss 3.0.3/Jetty, and copy the org.mortbay.j2ee.jar into your server's lib/ directory.