mm/memremap_pages: Kill unused __devm_memremap_pages()

Kill this definition that was introduced in commit 41e94a85 ("add
devm_memremap_pages") add never used.

Cc: Christoph Hellwig <hch@lst.de>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/158041476158.3889308.4221100673554151124.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>