[SCSI] aic7xxx: Remove slave_destroy

This is a cross-port from aic79xx; we still hit the occasional
BUG_ON in slave_destroy. And again we don't really need the
slave_destroy callback nor the ahc_linux_target structure
at all.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>