[!!!][TASK] Remove SU mode "change to" in EXT:beuser

This patch removes the ability to switch to a user permanently.
The user now always gets an "exit" button offered to switch back.

Resolves: #62595
Releases: master
Change-Id: I08925c5249aca30225016ffb047be09b5f48b338
Reviewed-on: http://review.typo3.org/33688
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>