gazebo_ros_utils: don't set tf_prefix if empty (ros-simulation#1173)

Similar changes were made in ros-simulation#1143 to address ros-simulation#554,
but this class wasn't included.

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

gazebo_plugins/src/gazebo_ros_utils.cpp

@@ -85,11 +85,6 @@ void GazeboRos ::readCommonParameter() {
 
 
     tf_prefix_ = tf::getPrefixParam(*rosnode_);
-    if(tf_prefix_.empty())
-    {
-        tf_prefix_ = namespace_;
-        boost::trim_right_if(tf_prefix_,boost::is_any_of("/"));
-    }
     ROS_INFO_NAMED("utils", "%s: <tf_prefix> = %s", info(), tf_prefix_.c_str());
 }