Cleanup of older OVAL definitions files

Hello,

since #864761 was fixed earlier this week, the OVAL definitions files
have changed substantially: we now generate definitions per release
instead of per year, so I'd like to have the now-obsolete
oval-definitions-[12]*.xml removed from
https://www.debian.org/security/oval.

Is this something the web team can do ? The "clean" Makefile target
could take care of that (but would also remove
oval-definitions-<release>.xml until "install" gets called again in the
next cron).

Cheers,

--Seb