revert(experiment-platform): remove backend-sealed-secret from kustomization

Sealed secret is not ready yet (placeholder). Will re-add after
R2 token is issued and seal.sh is executed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: soohyunme

services/experiment-platform-pseudolab/overlays/prod/kustomization.yaml

@@ -6,7 +6,6 @@ resources:
   - namespace.yaml
   - ingress.yaml
   - ghcr-pull-secret.yaml
-  - backend-sealed-secret.yaml
 commonLabels:
   environment: production
   project: experiment-platform