[BUGFIX] rsaauth: remove session-cookie when no longer used

When deleting the rsa key from the database we now remove the PHP cookie
as well in order to keep the system cleaned up.

Change-Id: I037deabf0a60e6ad785678b60424b3cc49af77b0
Resolves: #61016
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/32212


Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>