scsi: sg: Remove unnecessary NULL check before unregister_sysctl_table()

unregister_sysctl_table() checks for NULL pointers internally.  Remove
unneeded NULL check here.

Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Link: https://lore.kernel.org/r/20250514032845.2317700-1-nichen@iscas.ac.cn


Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>