NEXUS-5348: purge persist data too

PurgeTimeline task was only purging lucene index, but not the persisted data. The latter may consume huge amount of diskspace. With this change, both places. index and persisted files are purged.

Issue:
https://issues.sonatype.org/browse/NEXUS-5348

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF23