dma-mapping.h, x86: remove last user of dma_mapping_ops->map_simple

pci-dma.c doesn't use map_simple hook any more so we can remove it
from struct dma_mapping_ops now.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>