Removes deprecated github.del_branch_on_merge setting (apache#3670)

The `github.del_branch_on_merge` setting was renamed to `github.pull_requests.del_branch_on_merge`.

Author: ppkarwasz

.asf.yaml

@@ -40,7 +40,6 @@ github:
     issues: true
     discussions: true
     projects: true
-  del_branch_on_merge: true
   autolink_jira:
     - LOG4J2
   labels: