Block Aperture Window support was an attempt to layer an error model
over PMEM for platforms that did not support machine-check-recovery.
However, it was abandoned before it ever shipped, and only ever existed
in the ACPI specification. Meanwhile Linux has carried a large pile of
dead code for non-shipping infrastructure. For years it has been off to
the side out of the way, but now CXL and recent directions with DAX
support have the potential to collide with this code.

In preparation for adding discontiguous namespace support, a
pre-requisite for the nvdimm subsystem to replace device-mapper for
striping + concatenation use cases, delete BLK aperture support.

On the obscure chance that some hardware vendor shipped support for this
mode, note that the driver will still keep BLK space reserved in the
label area. So an end user in this case would still have the opportunity
to report the regression to get BLK-mode support restored without
risking the data they have on that device.
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/164688416668.2879318.16903178375774275120.stgit@dwillia2-desk3.amr.corp.intel.comSigned-off-by: NDan Williams <dan.j.williams@intel.com>