powerpc: Remove dead BIO_VMERGE_BOUNDARY definition

The block layer dropped the virtual merge feature
(b8b3e16c).  BIO_VMERGE_BOUNDARY
definition is meaningless now (For POWER, BIO_VMERGE_BOUNDARY has been
meaningless for a long time since POWER disables the virtual merge
feature).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>