GitHub Actions: Don't build offline docs on pushes and pull requests

This was a leftover from the pull request.

.github/workflows/build_offline_docs.yml

@@ -1,7 +1,5 @@
 name: Build documentation for offline usage
 on:
-  push: # FIXME
-  pull_request: # FIXME
   schedule:
     # Every week on Monday at midnight (UTC).
     # This keeps the generated HTML documentation fresh.