[BUGFIX] Don't set default start module in UC explicitly

The backend already provides the possibility for a backend user to use
the first available module as "start module". However, if the UC of
backend user is empty, its start module is set to "help_AboutAbout"
explicitly, which contradicts the mentioned feature. Additionally,
EXT:about is an optional dependency which may break the backend at this
point.

Because of the listed reasons, explicitly setting "help_AboutAbout" has
been removed as well as the override by EXT:dashboard.

Resolves: #91127
Releases: master
Change-Id: I04fb79f8b59f7e8a07736ffc0d328979075f09a4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64247


Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Richard Haeser <richard@maxserv.com>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Richard Haeser <richard@maxserv.com>