[BUGFIX] Fix handling of Storage.Client

The "Storage" module gets the missing "unset" method to remove values
from localStorage. The "isset" method is fixed to determine whether
a value is really existing.

Resolves: #82613
Releases: master, 8.7
Change-Id: Icb4b862b380811a7ba7f897be3c4809b718d2810
Reviewed-on: https://review.typo3.org/54229


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>