Suggestion: remove urdf files from support pkgs

[gavanderhoorn]

Quite a few robot support packages provide both .xacro as well as .urdf files for the variants they support.

While that made sense in the early days, xacro has stabilised to such a degree that I believe these are not longer needed.

They increase the maintenance burden (see #234 fi, where both the .xacro and the .urdfs needed to be updated), are not composable like .xacros are, and are also not used by the moveit configs (except the one for the SIA20D).

Similar to other ROS-I repositories, I propose we remove the .urdf files and instead rely fully on the .xacros.