s390/dasd: remove unnecessary null test before debugfs_remove

This fixes checkpatch warning:
"WARNING: debugfs_remove(NULL) is safe this check is probably not required"

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Stefan Haberland <stefan.haberland@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>