Remove backport-specific Dependabot config (v0.34.x). (#8519)

After #8518, this separate configuration is no longer needed.
The master copy will target updates to this branch.

.github/dependabot.yml

@@ -1,27 +0,0 @@
-version: 2
-updates:
-  - package-ecosystem: github-actions
-    directory: "/"
-    schedule:
-      interval: daily
-      time: "11:00"
-    open-pull-requests-limit: 10
-  - package-ecosystem: npm
-    directory: "/docs"
-    schedule:
-      interval: daily
-      time: "11:00"
-    open-pull-requests-limit: 10
-    reviewers:
-      - fadeev
-  - package-ecosystem: gomod
-    directory: "/"
-    schedule:
-      interval: daily
-      time: "11:00"
-    open-pull-requests-limit: 10
-    reviewers:
-      - melekes
-      - tessr
-    labels:
-      - T:dependencies