clean cmake and package.xml for uuid_msgs

[mikaelarguedas]

I came across this when trying to compile that repository with ROS 2.

This is mostly simplifying the CMake logic for the message package.

I didn't target making unique_id compile as this is a pretty unrelated change and will make reviewing more difficult.

@matt-attack as the original contributor of the ROS2 port, can you confirm that this still works for your use case?

[mikaelarguedas]

these now live at https://github.com/ros2/unique_identifier_msgs/