Remove support for japicmp from Gradle build

Remove support for japicmp from Gradle build

I now spent a few hours trying to make it work and the Gradle support is
simply not production-ready and also not maintained, it triggers various
strange errors and does not supporting the usual Gradle conventions.

So I do not want to spend more time on it, feel free to revive it if you know how
to make this work properly.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1888138 13f79535-47bb-0310-9956-ffa450edef68