hirionx_ros_bridge.launch: use default for arg USE_SERVOCONTROLLER

current implementation failed with following error
```
while processing /home/leus/ros/hydro/src/rtm-ros-robotics/rtmros_hironx/hironx_ros_bridge/launch/hironx_ros_bridge.launch:
Invalid <arg> tag: cannot override arg 'USE_SERVOCONTROLLER', which has already been set. 

Arg xml is <arg name="USE_SERVOCONTROLLER" value="false"/>
```