Remove from docs

Author: christophfroehlich

doc/controllers_index.rst

@@ -51,7 +51,6 @@ The controllers are using `common hardware interface definitions`_, and may use
    Admittance Controller <../admittance_controller/doc/userdoc.rst>
    Effort Controllers <../effort_controllers/doc/userdoc.rst>
    Forward Command Controller <../forward_command_controller/doc/userdoc.rst>
-   Gripper Controller <../gripper_controllers/doc/userdoc.rst>
    Joint Trajectory Controller <../joint_trajectory_controller/doc/userdoc.rst>
    Parallel Gripper Controller <../parallel_gripper_controller/doc/userdoc.rst>
    PID Controller <../pid_controller/doc/userdoc.rst>