Revert "[FEATURE] Add userGroups to backendUser model"

This reverts commit ba746497220ad5f7244415742c171bb6f8143d3c

Backend user groups are a graph in CMS core. This is not reflected
by the current implementation. The feature is reverted to not add
a half baked solution.

Change-Id: I3b7ee94d5bfe72cc7530144bb681e8a913786a16
Releases: 6.2, 6.1
Related: #39329
Reviewed-on: https://review.typo3.org/20298
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn