block: remove unused REQ_TYPE_LINUX_BLOCK

Nobody uses REQ_TYPE_LINUX_BLOCK (and its REQ_LB_OP_*).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>