MAINT: Remove cell_metadata_filter: -all from template

[maxbane]

Before this merge, jupyter-book create mynewbook/ generates an example MyST notebook with following preamble:

---
jupytext:
  cell_metadata_filter: -all
  formats: md:myst
  text_representation:
    extension: .md
    format_name: myst
    format_version: 0.13
    jupytext_version: 1.11.5
kernelspec:
  display_name: Python 3
  language: python
  name: python3
---

This PR removes the line cell_metadata_filter: -all.

It seems extraneous, and causes unexpected behavior, especially for new users of jupyter-book, who are likely to use this file as the copy-and-paste basis for all of their MyST markdown notebooks. If they do so, they will find that when they use jupytext to sync between MyST and other representations of the notebook (e.g. ipynb for WYSIWYG editing), all cell metadata, including tags, are dropped when syncing. See for example the confusion here: mwouts/jupytext#1024

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[davidvandebunte]

It looks like @choldgraf added the file, so perhaps he recalls the motivation for the line?

According to Metadata filtering — Jupytext documentation even without cell_metadata_filter we get some sane filtering now. Perhaps that wasn't the case in the past.

[choldgraf]

This makes sense to me - not sure why it was there in the first place, I bet it was boilerplate of some kind. Thanks for the fix!

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