[AI-88] llm: Remove .claude/CONTRIBUTING.md (#1571)

## 🎟️ Tracking

https://bitwarden.atlassian.net/browse/AI-88

## 📔 Objective

We're retiring the per-repo Claude context guide in favour of the single
org-level guide in `contributing-docs`, so this removes our copy
outright with **no replacement file and no pointer**. A full-worktree
sweep found nothing in this repo referencing it, so the diff is exactly
one deleted file. Part of a fleet-wide change tracked under AI-88.

## 🚨 Breaking Changes

None. This is a documentation-only deletion with no public API,
behavior, or configuration change.