Re-add "ant clean-jars" on root level

Although ivy sync=true seems to work in most cases, sometimes (especially when updating version numbers) the old JAR files are not nuked on resolve. I have no idea about why this is still a bug in ivy, but sync=true does not help in all cases.

It would be good to re-add the ant clean-jars target to the top-level build.xml, because this allows you to remove all JAR files and do a re-resolve.

The bug occurred to markrmiller@gmail.com and also to me this morning when Mark updated slf4j versions: If you run ant jar-checksums after updating the version numbers, it creates 2 checksum files (for the old and the new version). The call to ant check-svn-working-copy then does not find a violation, because the old version's checksum is still recreated. It only complains about the new one, but when you add it it will not complain anymore about the old one.

The violation is only found by Jenkins, because this one does a fresh svn checkout (or it emulates one), so it starts without any JARs. Because of this, checksum re-creation deletes the old file (still on subversion server) and a violation is detected.

Currently I do the JAR file deletion by a find/xargs/rm and then recreate checksums, which finds the problem. The good old ant clean-jars would be back as a last resort if you are hit my this IVY bug when you are updating revision numbers.