Fix alteration of new router parameters

If a router parameter has no default value, the previous value would be
returned as an empty string. A debug assertion would be triggered when a
parameter of this type was altered.

When a new router parameter is encountered and the alteration fails, the
modified value in the service need to be removed. Previously, the new
value would have been stored in the service with an empty value which
would have caused problems.

server/core/internal/service.h

@@ -135,6 +135,14 @@ void  service_update(SERVICE *service, char *router, char *user, char *auth);
  */
 void service_add_parameters(SERVICE *service, const MXS_CONFIG_PARAMETER *param);
 
+/**
+ * @brief Remove service parameter
+ *
+ * @param service Service to modify
+ * @param key     Parameter to remove
+ */
+void service_remove_parameter(SERVICE *service, const char* key);
+
 /**
  * @brief Replace service parameter
  *