* resurrect lost purge broken files scheduler tasks

* some cleanup in comments and user facing text
* inline simple private method used only once

in https://github.com/sonatype/nexus-ruby-support/issues/55#issuecomment-60445832 I had the feeling it is not clear which scheduled task is meant for the hosted repo or for the proxied repo. this is meant to help to make the intension clearer - I hope.

somehow - properly I wanted to delete it - the purge-broken-files tasks is not there anymore. this patch will resurrect it, since the under lying code still exists and gets tested.
lets not leak in stuff like this please, things like this have a habit of becoming stale.