[SCSI] mac_scsi: Remove unused variable default_instance

This fixes:

drivers/scsi/mac_scsi.c:220:5: warning: "NDEBUG_ABORT" is not defined
drivers/scsi/mac_scsi.c:271:5: warning: "NDEBUG_ABORT" is not defined

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>