[BUGFIX] rsaauth BackendFactory does not unset backend

The BackendFactory of rsaauth tries to forcefully unset an
instantiated backend, but unsets the class name instead of the object.

Resolves: #50411
Releases: 6.2, 6.1, 6.0
Change-Id: I26712c73b74788cad159f95c7e54cbbbf69e8764
Reviewed-on: https://review.typo3.org/22571
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind