scsi: virtio_scsi: Remove unnecessary condition check

kmem_cache_destroy() and mempool_destroy() both correctly handle null
pointer parameters. There is no need to check if the parameter is null
before calling these functions.

Link: https://lore.kernel.org/r/1594307167-8807-1-git-send-email-xianting_tian@126.com


Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Xianting Tian <xianting_tian@126.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>