contents-generation never cleans up apt-ftparchive files for old series

lp_publish@cocoplum:/srv/launchpad.net/ubuntu-archive/contents-generation/ubuntu/dists$ ls -w80
dapper hardy-backports karmic-security oneiric
dapper-backports hardy-proposed karmic-updates oneiric-backports
dapper-proposed hardy-security lucid oneiric-proposed
dapper-security hardy-updates lucid-backports oneiric-security
dapper-updates intrepid lucid-proposed oneiric-updates
edgy intrepid-backports lucid-security precise
feisty intrepid-proposed lucid-updates precise-backports
feisty-backports intrepid-security maverick precise-proposed
feisty-proposed intrepid-updates maverick-backports precise-security
feisty-security jaunty maverick-proposed precise-updates
feisty-updates jaunty-backports maverick-security quantal
gutsy jaunty-proposed maverick-updates quantal-backports
gutsy-backports jaunty-security natty quantal-proposed
gutsy-proposed jaunty-updates natty-backports quantal-security
gutsy-security karmic natty-proposed quantal-updates
gutsy-updates karmic-backports natty-security
hardy karmic-proposed natty-updates

We only update this for supported series, and everything else is junk. It would be helpful for generate_contents_files.py to clean up the old detritus automatically, rather than requiring administrative intervention.