clean up data of products that do not exist anymore

[ghost]

right now, when products stop existing, we continue to have them on the page, they just are not linked from the overview anymore. it is a bit error-prone to manually clean up after them. maybe we could check for them whenever we build anything and clean up automatically and do a full sync in those cases.

on the other hand, we currently maintain redirects by hand and it would not be ideal if we delete content before we have redirects set up. quandary...