Bug#856693: Drop ant dependency

[Emmanuel Bourg]

The ant dependency is only used for the excelant tasks [1] (which are
never used in Debian [2]). Any use of these tasks implicitly means Ant
is already on the classpath. In this context Ant can be seen as a
runtime, much like the Servlet API for web based stuff. So I think it
makes sense to remove the dependency from the binary package.

Splitting ant into ant+libant-java is a good idea, but this can probably
wait for the Buster development cycle.

Emmanuel Bourg

[1] https://poi.apache.org/spreadsheet/excelant.html
[2] https://codesearch.debian.net/search?q=excelant