[TASK] Have correct blog_example related_posts TCA

Similar to the recently fixed other MM TCA
setup of blog_example, the "related_posts"
is adapted as well.

This removes the last mm table definition from
ext_tables.sql - the auto-creation kicks in as
expected now.

Resolves: #101537
Related: #101528
Releases: main, 12.4
Change-Id: Ia92ce6bbc78a493e1a2028bd9ab7a31952181dd3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80299


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>