Use S3 object expiry for OCSP snapshots

ocsp-responder/responder.example.yaml

@@ -11,8 +11,6 @@ s3:
   # endpoint: https://s3.example.com
   # force_path_style: true
   cache_ttl_ms: 1000
-  # Hard maximum snapshot age. Keep comfortably above KCA's heartbeat interval.
-  stale_grace_seconds: 60
 
 issuers:
   - name: Example Root CA