[IA64] remove dead BIO_VMERGE_BOUNDARY definition

The block layer dropped the virtual merge feature
(b8b3e16c

). BIO_VMERGE_BOUNDARY
definition is meaningless now (For IA64, BIO_VMERGE_BOUNDARY has been
meaningless for a long time since IA64 disables the virtual merge
feature).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>