FEAT: remove tags from notebook cell if empty

[redeboer]

Closes #349

---

Added the following hook:

repos:
  - repo: https://github.com/ComPWA/policy
    rev: ""
    hooks:
      - id: remove-empty-tags

This removes the tags cell metadata field from Jupyter notebooks if it is empty, e.g.:

"metadata": {
 "tags": []
}

"metadata": {}

---

Also removing slideshow from the configure cell from set-nb-cells in order to address ComPWA/qrules#277.