Fix dependabot file

We're trying it out with the defaults to start with, so let's not
accidentally make two of them.

Author: mhagander

.github/dependabot.yml

@@ -2,8 +2,6 @@ version: 2
 updates:
   - package-ecosystem: "gradle"
     directory: "/"
-    registries:
-      - maven-google
     schedule:
       interval: "daily"
     open-pull-requests-limit: 10