Impact: cleanup, remove dead code

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

BIO_VMERGE_BOUNDARY definition is meaningless now.
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: NJens Axboe <jens.axboe@oracle.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>