remove explicit dependency on urdfdom

`urdfdom` is provided via `urdf` and so the `catkin_*` CMake variables replace the `urdfdom_*` variables. The current setup was unbalanced anyways because along with `urdfdom`, `urdfdom_headers` should have been being depended on and used. This precipitated from `urdfdom`'s rosdep key changing as it became a system dependency in Indigo, whereas before it was distributed as a ROS package.