remove explicit dependency on urdfdom

urdfdom is provided via urdf and catkin_* CMake
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.
Merge pull request #786 from ros-visualization/indigo_remove_explicit_dep_on_urdfdom

remove explicit dependency on urdfdom