alpha: remove dead BIO_VMERGE_BOUNDARY

The block layer dropped the virtual merge feature
(b8b3e16c). 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>