ci: remove unused GitHub Action file

The `workflow_run` event requires the executed workflow files exist in
the default branch.

.github/workflows/deploy-hitl-frontend.yml

@@ -1,25 +0,0 @@
-name: Deploy HITL frontend
-
-on:
-  workflow_run:
-    workflows: ["Build and Push API & Web"]
-    branches:
-      - "feat/hitl-frontend"
-    types:
-      - completed
-
-jobs:
-  deploy:
-    runs-on: ubuntu-latest
-    if: |
-      github.event.workflow_run.conclusion == 'success' &&
-      github.event.workflow_run.head_branch == 'feat/hitl-frontend'
-    steps:
-      - name: Deploy to server
-        uses: appleboy/ssh-action@v0.1.8
-        with:
-          host: ${{ secrets.HITL_SSH_HOST }}
-          username: ${{ secrets.SSH_USER }}
-          key: ${{ secrets.SSH_PRIVATE_KEY }}
-          script: |
-            ${{ vars.SSH_SCRIPT || secrets.SSH_SCRIPT }}