Use Dependabot's default identity at GitHub

A personal token (PAT) is the only mechanism available to raise Dependabot's API rate limit in a GitHub project. We don't want that right now, so this reverts 5b7538a and 2697e42. Issue: PGO-2563

Author: cbandy

.github/dependabot.yml

@@ -13,16 +13,6 @@
 ---
 version: 2
 
-registries:
- # Authenticate to GitHub for higher API rate limits
- # https://docs.github.com/en/rest/using-the-rest-api/rate-limits-for-the-rest-api
- # https://docs.github.com/en/code-security/dependabot/working-with-dependabot/configuring-access-to-private-registries-for-dependabot#git
- github:
- type: git
- url: https://github.com
- username: x-access-token
- password: ${{ secrets.DEPENDABOT_TOKEN }}
-
 updates:
  - package-ecosystem: github-actions
  directories: