[TASK] Remove last part of singleton pattern in getUserObj()

In 8.0, the logic for having getUserObj() called with a "&" to
implement a singleton pattern has been removed, however
some code is still left and now unused.

Resolves: #75767
Releases: master
Change-Id: I76d5eb211662b60d9b34b38b3c0efb77a3b33fc4
Reviewed-on: https://review.typo3.org/47771


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>