parisc: remove dead BIO_VMERGE_BOUNDARY and BIO_VMERGE_MAX_SIZE definitions

The block layer dropped the virtual merge feature
(b8b3e16c

). BIO_VMERGE_BOUNDARY and
BIO_VMERGE_MAX_SIZE definitions are meaningless now.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>