mm/hmm: remove hmm_range_dma_map and hmm_range_dma_unmap

These two functions have never been used since they were added.

Link: https://lore.kernel.org/r/20191113134528.21187-1-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>