ros time source should not use parameter client

[wjwwood]

Right now the TimeSource class implementation in rclcpp uses a ParameterClient to get notifications of when a parameter changes, which:

• does not work if parameter service is disabled in the node
• is not required, the node knows when the parameter changes (with or without ROS services)

[wjwwood]

rclcpp/rclcpp/src/rclcpp/time_source.cpp

Lines 69 to 77 in 33a755c

	parameter_client_ = std::make_shared<rclcpp::AsyncParametersClient>(
	node_base_,
	node_topics_,
	node_graph_,
	node_services_
	);
	parameter_subscription_ =
	parameter_client_->on_parameter_event(std::bind(&TimeSource::on_parameter_event,
	this, std::placeholders::_1));

[CursedRock17]

Since pull request #740 was merged, time_source no longer uses parameter_client_, so this issue should have been solved correct? Otherwise I'd happy to make some changes needed.

[clalancette]

Yeah, I'm going to close this one out for now. We may have to make further changes to the TimeSource (see the conversation in ros2/geometry2#630), but that is not what this PR is talking about.