Remove gripper_controllers dependency (#3474)

The package got removed upstream and it seems not to be used in this
package, anyway.

Author: fmauch

moveit_ros/moveit_servo/package.xml

@@ -39,7 +39,6 @@
   <depend>trajectory_msgs</depend>
 
   <exec_depend>controller_manager</exec_depend>
-  <exec_depend>gripper_controllers</exec_depend>
   <exec_depend>joint_state_broadcaster</exec_depend>
   <exec_depend>joint_trajectory_controller</exec_depend>
   <exec_depend>joy</exec_depend>