ppc/spapr: remove deprecated machine pseries-2.5

ppc/spapr: remove deprecated machine pseries-2.5

Commit 1392617d3576 intended to tag pseries-2.1 - 2.11 machines as
deprecated with reasons mentioned in its commit log.
Removing pseries-2.5 specific code with this patch for now.

Also drop sPAPRMachineClass::use_ohci_by_default which is now useless.

Suggested-by: Cédric Le Goater <clg@kaod.org>
Reviewed-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>