How to remove the namespace of tf published by gazebo sensor plugin?

edit

I spawn a robot called robuster, which has a ray sensor. The sensor's gazebo reference is "livox_base".However, in tf_tree, gazebo publish a "robuster::livox_base" tf so that the tree has two roots. How to remove the namespace "robuster?"

As the suggestion by skpro19, it's my launch file

<launch>
<arg name="model" default="waffle_pi" doc="model type [burger, waffle, waffle_pi]"/>
<arg name="x_pos" default="1"/>
<arg name="y_pos" default="-8"/>
<arg name="z_pos" default="0.0"/>
<arg name="roll" default="0"/>
<arg name="pitch" default="0"/>
<arg name="yaw" default="1.57"/>
<arg name="use_ekf_localization" default="false" />
<arg name="world" default="$(find active_mapping)/worlds/house.world" />
<arg name="robot_urdf" default="$(find active_mapping)/urdf/robuster_horizon.urdf.xacro" />
<remap to="cmd_vel" from="mr1000_velocity_controller/cmd_vel"/>
<!-- <remap to="odom" from="mr1000_velocity_controller/odom"/> -->
<include file="$(find gazebo_ros)/launch/empty_world.launch">
<arg name="world_name" value="$(arg world)"/>
<arg name="paused" value="false"/>
<arg name="use_sim_time" value="true"/>
<arg name="gui" value="true"/>
<arg name="headless" value="false"/>
<arg name="debug" value="false"/>
</include>

<param name="robot_description" command="$(find xacro)/xacro --inorder $(arg robot_urdf)" />

<node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-urdf -model robuster -x $(arg x_pos) -y   $(arg y_pos) -z $(arg z_pos) -Y $(arg yaw) -param robot_description" />

<node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher">
  <param name="publish_frequency" type="double" value="30.0" />    
</node>


<rosparam file="$(find active_mapping)/config/robuster_horizon/crank_gazebo_control.yaml" command="load"/>
<rosparam file="$(find active_mapping)/config/robuster_horizon/diff_drive_control.yaml" command="load"/>
<node name="car_controller_spawner" pkg="controller_manager" type="spawner" ns="/"
  args="mr1000_velocity_controller crank_joint_position_controller mr1000_joint_publisher"/>

<group if="$(arg use_ekf_localization)">
  <node pkg="robot_localization" type="ekf_localization_node" name="ekf_localization">
    <rosparam command="load" file="$(find active_mapping)/config/robuster_horizon/localization.yaml" />
  </node>
</group>

<group unless="$(arg use_ekf_localization)">
  <node pkg="active_mapping" type="odom_to_tf" name="odom_to_tf">
  </node>
</group>

   </launch>