alpha: remove dead BIO_VMERGE_BOUNDARY

The block layer dropped the virtual merge feature
(b8b3e16cfe6435d961f6aaebcfd52a1ff2a988c5). BIO_VMERGE_BOUNDARY
definition is meaningless now.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

arch/alpha/include/asm/io.h

@@ -96,9 +96,6 @@ static inline dma_addr_t __deprecated isa_page_to_bus(struct page *page)
 	return page_to_phys(page);
 }
 
-/* This depends on working iommu.  */
-#define BIO_VMERGE_BOUNDARY	(alpha_mv.mv_pci_tbi ? PAGE_SIZE : 0)
-
 /* Maximum PIO space address supported?  */
 #define IO_SPACE_LIMIT 0xffff