rtt_gazebo_activity: removed find_package(Orocos-KDL REQUIRED) from 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.
Merge pull request #12 from meyerj/removed-orocos-kdl-from-rtt-gazebo-activity

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