Remove duplication introduced by using a panda.urdf & panda.urdf.xacro

[sjahr]

To enable support for Ignition Gazebo, the panda_description package currently contains a panda.urdf and a newer panda.urdf.xacro, introduced by #178.
The XACRO has been created to support finding package resource files dynamically which is needed for Gazebo. Unfortunately, the URDF is still required by RobotModelTestUtils. There are two possible solutions to this problem:

• Enable xacro support in RobotModelTestUtils
• Parse the urdf for Gazebo with something like https://github.com/ros/resource_retriever/blob/ros2/resource_retriever/src/resource_retriever/__init__.py#LL43C15-L43C15