scsi: xen-scsifront: remove DISABLE_CLUSTERING

There is no such limitation in the protocol or implementation, so
remove it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>