ci(mergify): upgrade configuration to current format (#1700)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: mergify[bot]

.github/mergify.yml

@@ -28,7 +28,6 @@ pull_request_rules:
     conditions:
       - merged
     actions:
-      delete_head_branch:
       label:
         remove:
           - "merge-conflicts"