[TASK] Cleanup BackendUserAuthentication->unpack_uc()

The backendUser->uc() call is called with a default value,
which is unnecessary and "looks ugly" ;).

Additionally $BE_USER calls unpack_uc() itself, basically doing
a unserialize() before, even though unpack_uc() does the same
thing.

Resolves: #78658
Releases: master
Change-Id: I492326373b61dba5cba61f24e64b31fddf9a2142
Reviewed-on: https://review.typo3.org/50595


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>