remove needless ISA_DMA_THRESHOLD

Architectures don't need to define ISA_DMA_THRESHOLD anymore.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: James Bottomley <James.Bottomley@suse.de>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>