[TASK] Unify simulate user settings for Backend admins

The "Simulate user" functionality of the "User settings"
module for backend admins is UX wise a distaster: This
functionality is hidden behind the settings of the currently
logged-in backend user in the toolbar, which then magically
allows to change foreign user settings.

A much better approach to administrate user settings of other
users is to fully switch to the target user using the
"Backend User" module.

Additionally, if an admin user changes specific user settings of
existing or new users over and over again (eg. backend language),
sane defaults should be configured via User TSconfig "setup."
instead.

The patch unifies this functionality by removing the additional
simulate backend user selector from user settings module.

Resolves: #85196
Releases: master
Change-Id: If0abf5a7ca3a62c669540e6ada3d61b08022fc09
Reviewed-on: https://review.typo3.org/57159


Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>