Remove FlavorNotFound dead code condition in API.resize

The condition removed in this change is dead code because
of the block above where new_instance_type is set. If a new
flavor is specified (resize) then flavors.get_flavor_by_flavor_id
will either find it or raise FlavorNotFound.

Change-Id: If66a28c07c619cb89d77ee926f60b5d48cbfd351

Author: mriedem

nova/compute/api.py

@@ -3652,9 +3652,6 @@ def resize(self, context, instance, flavor_id=None, clean_shutdown=True,
                     reason = _('Resize to zero disk flavor is not allowed.')
                     raise exception.CannotResizeDisk(reason=reason)
 
-        if not new_instance_type:
-            raise exception.FlavorNotFound(flavor_id=flavor_id)
-
         current_instance_type_name = current_instance_type['name']
         new_instance_type_name = new_instance_type['name']
         LOG.debug("Old instance type %(current_instance_type_name)s, "