[CLEANUP] Add phpDoc to properties in BackendController

Removes a unused property as well.

Resolves: #62627
Releases: master
Change-Id: I6ba4e7345b83dbf2aa66361f08dd27ad871541bd
Reviewed-on: http://review.typo3.org/33717
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>