s390/zcrypt: remove gratuitious NULL check in .remove() callbacks

As .remove() is only called after a successful .probe() call, we can
trust that the drvdata is valid.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>