405 when deleting infractions that fail to apply

[MarkKoz]

Sentry Issue: BOT-6Q

The infractions viewset doesn't support deleting infractions, but it's done here:

https://github.com/python-discord/bot/blob/a1a44be2b57e35fbaee8cac024fdd74c218c72b1/bot/cogs/moderation/scheduler.py#L173-L180

This is to delete infractions when they fail to apply on the Discord side of things. Should DELETE support be added to the API or can we re-order things to not have to later delete infractions? #986 is very relevant to this.