mm: remove CONFIG_MIGRATE_VMA_HELPER

CONFIG_MIGRATE_VMA_HELPER guards helpers that are required for proper
devic private memory support.  Remove the option and just check for
CONFIG_DEVICE_PRIVATE instead.

Link: https://lore.kernel.org/r/20190814075928.23766-11-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>