Removed find_package(Orocos-KDL REQUIRED) from rtt_gazebo_activity/CMakeLists.txt

... as KDL is not used in rtt_gazebo_activity.

These lines have been added during the merge in https://github.com/jhu-lcsr/rtt_gazebo/commit/f3b99b76a1404b46ea66bd15f0dd02e5e7b25fba.