scsi: xen-scsiback: Remove default fabric ops callouts

Remove callouts that are identical to the default implementations in TCM
Core.

Acked-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Dmitry Bogdanov <d.bogdanov@yadro.com>
Link: https://lore.kernel.org/r/20230313181110.20566-10-d.bogdanov@yadro.com


Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>