Remove unneeded taskdef for <setproxy> task.

The announcement for Ant 1.8.2 said,
* distinction between core tasks and optional tasks is abolished"
[http://mail-archives.apache.org/mod_mbox/ant-dev/201012.mbox/%3C4D18C53F.3060700%40gmx.de%3E]

Merged r1640642 from tomcat/tc8.0.x/trunk.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1640643 13f79535-47bb-0310-9956-ffa450edef68