Revert catkin detection

This pull request reverts @jbohren's b7a2c84395fc73f17400e6e2b9c8df7ad2edbb93.

There are cases where you have a package.xml, but do not want to build with catkin-support. This wouldn't be a big problem, if catkin support would not disable auto linking now since 071e19778bd4fb5fc5abc1246e435bfd6fc35de8. So even without having anything with catkin in the CMakeLists.txt file `orocos_use_package()` does not behave as expected anymore if there is a package.xml file present.

I would propose to just enforce that the user has to `find_package(catkin)` before the inclusion of the `UseOROCOS-RTT.cmake` script. What do you think?