🐛 e2e: skip clusterobjectset cleanup when BoxcutterRuntime is disabled (#2662)

Scenario cleanup unconditionally attempted to delete the pre-generated
clusterobjectset resource, but the CRD only exists when BoxcutterRuntime
is enabled. Guard the deletion with a feature gate check.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>