docs(artifact-registry): drop the slug availability check from ADR-009

Reverts the slug availability endpoint added in !20714 (merged) (merged earlier today, before review feedback on the companion internal MR landed).

The settled activation design (gitlab-org/gitlab#593818) validates format inline and availability at form submission, so atomic provisioning's refusal is the availability answer and a pre-submit check endpoint has no caller. Companion trim: internal-handbook!8882 (ADR-015 registration flow).

Related to gitlab-org/ops/artifact-registry#310 (closed)