ci: don't label Dependabot pull requests with 'automerge' anymore

Kodiak is configured to merge patch dependencies so the label is
no longer necessary and was causing Kodiak to merge all Dependabot
pull requests

Author: jrolfs

.github/dependabot.yml

@@ -4,7 +4,6 @@ updates:
     directory: '/'
     labels:
       - dependencies
-      - automerge
     schedule:
       interval: daily
     open-pull-requests-limit: 10