ci: drop cargo-vet

The baseline audited nothing: audits.toml was empty, imports.lock pulled
in no external audit sets, and all 780 dependencies sat under blanket
exemptions. Its only effect was failing CI whenever the dependency tree
moved, which is what the faultbox additions tripped over.

The workflow's other step, the advisory-ignore gate, already runs in
static-gates.yml, so nothing is lost with the file. `cargo deny check`
in test.yml still covers advisories, licenses, bans, and sources.

The per-crate `audit-as-crates-io = true` policies were wrong besides:
they demanded safe-to-deploy audits of this workspace's own crates.

Author: farhan-syah