Fix ai-traffic GUI checkbox

Setting was lost on sim-reset and restart
/sim/ai-traffic props are entirely obsolete (to be removed after 2.8)

gui/dialogs/atc-ai.xml

@@ -31,26 +31,9 @@
 			<checkbox>
 				<halign>left</halign>
 				<label>Enable AI traffic</label>
-				<property>/sim/ai-traffic/enabled</property>
+				<property>/sim/traffic-manager/enabled</property>
 			</checkbox>
 
-			<group>
-				<layout>hbox</layout>
-
-				<text>
-					<halign>left</halign>
-					<label>AI traffic density:</label>
-				</text>
-
-				<combo>
-					<halign>left</halign>
-					<property>/sim/ai-traffic/level</property>
-					<value>1</value>
-					<value>2</value>
-					<value>3</value>
-				</combo>
-			</group>
-
 		</group>
 
 		<empty>

gui/dialogs/scenario.xml

@@ -54,6 +54,7 @@
     <label>Enable AI traffic</label>
     <name>enable-ai-traffic</name>
     <property>/sim/traffic-manager/enabled</property>
+    <live>true</live>
     <binding>
         <command>dialog-apply</command>
         <object-name>enable-ai-traffic</object-name>

preferences.xml

@@ -689,12 +689,13 @@ Started September 2000 by David Megginson, david@megginson.com
 		</atc>
 
 		<ai-traffic>
+			<!-- Obsolete and unused - to be removed -->
 			<enabled type="bool" userarchive="y">false</enabled>
 			<level type="int" userarchive="y">1</level>
 		</ai-traffic>
 
 		<traffic-manager>
-			<enabled type="bool">true</enabled>
+			<enabled type="bool" userarchive="y">true</enabled>
 			<heuristics type="bool">true</heuristics>
 			<datafile type="string"></datafile>
 			<instantaneous-action type="bool">false</instantaneous-action>