Delete notifications for replies when the comment/post has been deleted

[Nukrid]

Currently, the GMT is facing a little bit of an issue with users posting bad comments on beatmaps offending others directly and swiftly deleting those, as we don't usually act on deleted content that has been deleted before we got there (unless it is very prominent and can clearly be seen as abuse), there are instances where the user deleted the comment itself, but the reply notification still appears for whoever posted the original comment, leading to some of the recent targeted harassment cases where they abuse the notification to send the message without getting punished.

This also applies to beatmap discussions and forum posts you are subscribed to, so they all need a treatment.

We could also just judge those cases one by one, but that adds a lot of overhead both for the moderation team and the user itself that has to actively report such conduct directly to someone on the team.

A simple solution would be to just delete the notification if the comment has been deleted, so that loophole can't really be exploited and the target won't even know anything happened.