mm/hmm: remove unused code and tidy comments

Delete several functions that are never called, fix some desync between
comments and structure content, toss the now out of date top of file
header, and move one function only used by hmm.c into hmm.c

Link: https://lore.kernel.org/r/20200327200021.29372-4-jgg@ziepe.ca


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