remove Briefkasten (unmaintained since May 2025) (#2441)

- ref: #1
- `ERROR:awesome_lint.py: Briefkasten: last updated -366 days, 0:46:29.494172 ago, older than 365 days`
- Does not use a release system

```
ghcr.io/tempoworks/txtdot:main (alpine 3.21.3)

Total: 50 (UNKNOWN: 0, LOW: 5, MEDIUM: 26, HIGH: 15, CRITICAL: 4)
```

Author: Rabenherz112