ppc/spapr: remove deprecated machine pseries-2.12

ppc/spapr: remove deprecated machine pseries-2.12

Commit 0cac0f1b964 marked pseries-2.12 machines as deprecated
with reasons mentioned in its commit log.
Removing pseries-2.12 specific code with this patch.

While at it, also remove pre-3.0-migration hacks introduced for backward
compatibility which are now turned useless.

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