The last line is to remove the previous version, since only the most recent version on a particular branch should be in the dist directory (older versions are archived automatically, see http://archive.apache.org/dist/bigtop/ and http://www.apache.org/dev/mirrors.html).
This page describes how to make a release of Apache Bigtop. Most of the content is shamelessly stolen from the the Whirr release process – thanks Whirr team!
...
Add the next version number (e.g. 0.2.0 after 0.1.0) to JIRA using this
link: https://issues.apache.org/jira/secureplugins/servlet/project/ManageVersions.jspa?pid=12311420.-config/BIGTOP/administer-versions?status=unreleased
In JIRA mark the released version as "released" on the "manage versions" page. Be sure to fill in a date if not already specified.
...