Use conditional in launch instead of group conditional

Signed-off-by: Aaron Chong <aaronchongth@gmail.com>

free_fleet_adapter/launch/fleet_adapter.launch.xml

@@ -8,29 +8,16 @@
   <arg name="server_uri" default="" description="The URI of the api server to transmit state and task information."/>
   <arg name="output" default="screen"/>
 
-  <!-- Fleet adapter -->
-  <group>
-    <group if="$(var use_sim_time)">
-      <node pkg="free_fleet_adapter"
-            exec="fleet_adapter.py"
-            args="-c $(var config_file) -n $(var nav_graph_file) -s $(var server_uri) -sim"
-            output="both">
-
-        <param name="use_sim_time" value="$(var use_sim_time)"/>
-        <param name="server_uri" value="$(var server_uri)"/>
-      </node>
-    </group>
+  <let name="use_sim_time_flag" if="$(var use_sim_time)" value="-sim"/>
+  <let name="use_sim_time_flag" unless="$(var use_sim_time)" value="" />
 
-    <group unless="$(var use_sim_time)">
-      <node pkg="free_fleet_adapter"
-            exec="fleet_adapter.py"
-            args="-c $(var config_file) -n $(var nav_graph_file) -s $(var server_uri)"
-            output="both">
-
-        <param name="use_sim_time" value="$(var use_sim_time)"/>
-        <param name="server_uri" value="$(var server_uri)"/>
-      </node>
-    </group>
-  </group>
+  <!-- Fleet adapter -->
+  <node pkg="free_fleet_adapter"
+        exec="fleet_adapter.py"
+        args="-c $(var config_file) -n $(var nav_graph_file) -s $(var server_uri) ${var use_sim_time_flag)"
+        output="both">
+    <param name="use_sim_time" value="$(var use_sim_time)"/>
+    <param name="server_uri" value="$(var server_uri)"/>
+  </node>
 
 </launch>