ppc: Remove some stub POWER6 models

The CPU model table includes stub (commented out) definitions for
CPU_POWERPC_POWER6_5 and CPU_POWERPC_POWER6A.  These are not real cpu
models, but represent the POWER6 in some compatiblity modes.  If we ever
do implement POWER6 (unlikely), we'll implement its compatibility modes in
a different way (similar to what we do for POWER7 and POWER8).  So these
stub definitions can be removed.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>