ant task to remove files from a jar

Have you tried using the zipfileset task?

<jar destfile="stripped.jar">
  <zipfileset src="full.jar" excludes="files/to/exclude/**/*.file" />
</jar>

Example

<property name="library.dir" value="dist" />
<property name="library.file" value="YourJavaArchive.jar" />
<property name="library.path" value="${library.dir}/${library.file}" />
<property name="library.path.new" value="${library.dir}/new-${library.file}" />

<target name="purge-superfluous">
  <echo>Removing superfluous files from Java archive.</echo>

  <jar destfile="${library.path.new}">
    <zipfileset src="${library.path}" excludes="**/ComicSans.ttf" />
  </jar>

  <delete file="${library.path}" />
  <move file="${library.path.new}" tofile="${library.path}" />
</target>