FIX: Don't try to start franka_gripper controller

[gollth]

Steps to reproduce

$ cd panda_moveit_config
$ git checkout noetic-devel
$ catkin build

$ roslaunch panda_moveit_config franka_control.launch robot_ip:=<MY-ROBOT>
...
[ERROR] Could not load controller 'franka_gripper' because controller type 'GripperCommand' does not exist.
...

Fix

The gripper controller is automatically started in franka_control (whenever use_gripper:=true), simply don't try to start it =)

[rhaschke]

Change is ok, but the explanation was misleading (for me):
Actually, 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).