pmem: kill memremap_pmem()

Now that the pmem-api is defined as "a set of apis that enables access
to WB mapped pmem",  the mapping type is implied.  Remove the wrapper
and push the functionality down into the pmem driver in preparation for
adding support for direct-mapped pmem.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>