ppc/spapr: remove deprecated machine pseries-2.7

ppc/spapr: remove deprecated machine pseries-2.7

Commit 1392617d3576 intended to tag pseries-2.1 - 2.11 machines as
deprecated with reasons mentioned in its commit log.
Removing pseries-2.7 specific code with this patch for now.

While at it, also remove pre-2.8-migration and pci/mmio hacks introduced
for backward compatibility.

Suggested-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>