[TASK] Simplify default backend layout

To simplify the life for beginners the default backend layout
should only show one content column.
Therefore only the main column is shown instead of 4 columns.

Resolves: #81109
Releases: master
Change-Id: Ide0d01fb99a191e35faa8beff4ac06d450c75dc0
Reviewed-on: https://review.typo3.org/52682


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>