Don't load/start franka_gripper as ROS controller (#99)

franka_ros doesn't provide a ros_control controller for the gripper, but a GripperCommand action server.
Thus, there is no need to load/start the gripper controller (via ros_control).

launch/ros_controllers.launch

@@ -6,5 +6,5 @@
 
   <!-- Load and start the controllers -->
   <node ns="/" name="controller_spawner" pkg="controller_manager" type="spawner" respawn="false" output="screen"
-        args="$(arg transmission)_joint_trajectory_controller franka_gripper" />
+        args="$(arg transmission)_joint_trajectory_controller" />
 </launch>