remove: cancel-on-merge workflow (#38)

The styfle/cancel-workflow-action is not compatible with Forgejo Actions,
causing failures with 'Cannot read properties of undefined (reading id)'.

The pr-tests.yaml workflow already has concurrency control that cancels
in-progress runs when new commits are pushed, which handles most cases.
Removing this workflow to eliminate CI errors.

Reviewed-on: graham/towerops-web#38

.forgejo/workflows/cancel-on-merge.yaml

@@ -1,20 +0,0 @@
-name: Cancel Tests on Merge
-
-on:
-  pull_request:
-    types: [closed]
-    branches:
-      - main
-      - production
-
-jobs:
-  cancel-tests:
-    name: Cancel Running Tests
-    runs-on: ubuntu-22.04
-    if: github.event.pull_request.merged == true
-    steps:
-      - name: Cancel in-progress workflow runs
-        uses: https://github.com/styfle/cancel-workflow-action@0.12.1
-        with:
-          workflow_id: pr-tests.yaml
-          access_token: ${{ github.token }}