chore(dependabot): remove the Dependabot configuration (#151)

Renovate is the dependency-update tool for this organization and it is
live here — Dependency Dashboard #5. Dependabot duplicated it: both bots
opened a pull request for the same update, and whichever merged first
left the other to be closed or auto-closed, costing CI minutes and
review attention twice.

Nothing loses coverage. Measured on this repository rather than assumed:

- the removed `.github/dependabot.yml` declared: `github-actions`
- Renovate currently detects: `docker-compose,dockerfile,github-actions`

Renovate also carries the policy the Dependabot block never had —
stability days, the automerge rules and the `deps-no-automerge` label
all come from `github>netresearch/renovate-config`.

**Dependabot security updates are unaffected.** Those are a repository
setting, not this file, and they stay on.

Part of a fleet-wide sweep for netresearch/.github#410, applied to the
78 non-mirror repositories where Renovate is demonstrably running and
covers every ecosystem Dependabot declared. Two repositories were
deliberately excluded because Renovate does not currently see an
ecosystem Dependabot does: `ldap-manager` (gomod) and
`node-agent-skill-coordinator` (github-actions).

_Assisted by claude-code:claude-opus-5 —
[Session](https://claude.ai/code/session_01AXRGd6GWAC7TLK3wuiYeGg)_

Author: CybotTM