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: