cpu: drop unnecessary NULL check and cpu_common_class_by_name()

both do nothing as for the first all callers
   parse_cpu_model() and qmp_query_cpu_model_()
should provide non NULL value, so just abort if it's not so.

While at it drop cpu_common_class_by_name() which is not need
any more as every target has CPUClass::class_by_name callback
by now, though abort in case a new arch will forget to define one.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1518013857-4372-1-git-send-email-imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>