PCI/P2PDMA: Remove pci_p2pdma_[un]map_sg()

This interface is superseded by support in dma_map_sg() which now supports
heterogeneous scatterlists. There are no longer any users, so remove it.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>