Revert "spapr: populate device tree depending on XIVE_EXPLOIT option"

This reverts commit b8768042.

I thought this was a harmless preliminary for XIVE enablement patches
we expect later on.  However, due to some subtle interactions between
qemu and SLOF (guest firmware) this breaks some things.  Revert it for
now, we'll work out how to fix it when the rest of the XIVE patches
are ready.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>