Update prd.yml

Author: NikolajBjorner

.github/workflows/prd.yml

@@ -2,7 +2,6 @@ name: genai pull request description
 on:
     pull_request:
         types: [ready_for_review, review_requested]
-    workflow_dispatch:
 concurrency:
     group: genai-pr-review-${{ github.workflow }}-${{ github.ref }}
     cancel-in-progress: true