Correctly delete xml file after restore and conversion to rrd

When doing "Generating RRD graphs" at bootup, the data is restored from /cf/conf/rrd.tgz into xml format files in /var/db/rrd. Those xml files are then convert to rrd files. After that, the xml files should be deleted - but the xml file path was not quite right, so they were not being deleted.
This fixes it.
Merge pull request #1356 from phil-davis/patch-3