ci: remove the vu1nz security scan (#265)

vu1nz reviews a diff by calling Claude, which needs ANTHROPIC_API_KEY
supplied through the ENV_FILE secret. That key is not present on this
repository, so the scanner has never reviewed a pull request. On pack
1.0.0 and 1.0.1 that failure was silent: the job reported "0 finding(s),
no high/critical issues" on a diff nothing had read, which is worse than
no scanner at all.

threatcrush-scan covers the same ground deterministically - credentials,
injection, SSRF, unsafe deserialisation, XXE, dependency tampering - with
no API key and no per-pull-request cost.

Reinstallable from the sh1pt Actions Store if the key is ever provisioned.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: ralyodio