Documentation: remove the PCI DMA API description in DMA-API.txt

- remove the PCI DMA API description in DMA-API.txt
- remove the descriptions of dma_unmap macros since
  PCI-DMA-mapping.txt has the same description.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: "David S. Miller" <davem@davemloft.net>
Reviewed-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>