QuantityValue: clean up data on deletion of units

[weisswurstkanone]

Check approach first:

• maintenance script or
• foreign key with delete

also see #3372

[stale[bot]]

Thanks a lot for reporting the issue. The issue was not considered by us as "Priority" or "Backlog", so we're not gonna work on that anytime soon. In case this is a bug report, please create a pull request fixing the issue, we'll then review it as soon as possible. If you're interested in contributing a feature, please contact us first here before creating a pull request, we'll then decide whether we'd accept it or not. Thanks for your understanding.