[CLEANUP] Unqoute integer in BackendUserSessionRepository

In the method "findByBackendUser" in BackendUserSessionRepository,
the beuser uid used in the WHERE condition is casted to int and is
then quoted again.
This is not necessary and breaks the SQL for some DBMS.

Resolves: #61743
Releases: master, 6.2
Change-Id: Ibff2b6189efa16b8d7ce0acbc1637ce374f5f24d
Reviewed-on: http://review.typo3.org/32899
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>