Remove reviewers from Dependabot configuration. (#6604)

The Dependabot PRs get assigned to mozilla-services/obs-team automatically
due to our branch protection rules, so we don't need a reviewer in the
Dependabot configuration. Moreover, the current configuration tried to
assign the PRs to https://github.com/obs-team, which is different from
https://github.com/orgs/mozilla-services/teams/obs-team.

.github/dependabot.yml

@@ -5,21 +5,15 @@ updates:
     directory: "/"
     schedule:
       interval: "monthly"
-    reviewers:
-      - "obs-team"
 
   - package-ecosystem: "docker"
     directory: "/docker"
     schedule:
       interval: "monthly"
-    reviewers:
-      - "obs-team"
 
   # Set update schedule for GitHub Actions
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
       interval: "monthly"
-    reviewers:
-      - "obs-team"
     rebase-strategy: "disabled"