Drop pureos-security-hardening

It was folded into pureos-settings for crimson.

Prompted by

removing pureos-security-hardening/0.2/amd64 from crimson makes <a href="#pureos-standard/0.12.0/amd64">pureos-standard/0.12.0/amd64</a> uninstallable

at https://master.pureos.net/migrations/excuse/f5e2f504-d0a7-4eb8-bf09-6dec148f43a9