ia64: remove support for the SGI SN2 platform

The SGI SN2 (early Altix) is a very non-standard IA64 platform that was
at the very high end of even IA64 hardware, and has been discontinued
a long time ago.  Remove it because there no upstream users left, and it
has magic hooks all over the kernel.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lkml.kernel.org/r/20190813072514.23299-16-hch@lst.de
Signed-off-by: Tony Luck <tony.luck@intel.com>