Remove unneeded try..except in heartbeat

as callback_url is now a required argument for this method,
there's no need for such check (presumably copy-pasted from
heartbeat-as-vendor-interface).

Besides, assigning a value to dictionary key never raises KeyError.

The now deprecated vendor heartbeat is unchanged.

Change-Id: Id1e1a15b2cbc5dfbae9655b0edbc30c0a0cf2c57

ironic/drivers/modules/agent_base_vendor.py

@@ -457,12 +457,7 @@ class AgentDeployMixin(object):
             {'node': node.uuid,
              'heartbeat': driver_internal_info.get('agent_last_heartbeat')})
         driver_internal_info['agent_last_heartbeat'] = int(time.time())
-        try:
-            driver_internal_info['agent_url'] = callback_url
-        except KeyError:
-            raise exception.MissingParameterValue(_('For heartbeat operation, '
-                                                    '"agent_url" must be '
-                                                    'specified.'))
+        driver_internal_info['agent_url'] = callback_url
 
         node.driver_internal_info = driver_internal_info
         node.save()