[Gameplay A24] - Permadeath for heroes.

Part of:
https://wildfiregames.com/forum/index.php?/topic/27214-borg-expansion-pack-mod-implementation-in-0ad-alpha-24-release/.
Refs. #1432

Differential revision: D3265
Reviewed by: @borg-, @Nescio
This was SVN commit r24496.

binaries/data/mods/public/simulation/templates/template_unit_hero.xml

@@ -13,8 +13,8 @@
     units/heroes/hero_garrison
   </Auras>
   <Cost>
-    <Population>2</Population>
-    <BuildTime>40</BuildTime>
+    <Population>0</Population>
+    <BuildTime>50</BuildTime>
     <Resources>
       <food>100</food>
       <metal>250</metal>
@@ -75,6 +75,7 @@
   </Sound>
   <TrainingRestrictions>
     <Category>Hero</Category>
+    <MatchLimit>1</MatchLimit>
   </TrainingRestrictions>
   <Vision>
     <Range>100</Range>

binaries/data/mods/public/simulation/templates/template_unit_hero_cavalry.xml

@@ -1,7 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Entity parent="template_unit_hero">
   <Cost>
-    <BuildTime>50</BuildTime>
     <Resources>
       <food>300</food>
       <wood>200</wood>

binaries/data/mods/public/simulation/templates/template_unit_hero_elephant_melee.xml

@@ -14,8 +14,6 @@
     </Melee>
   </Attack>
   <Cost>
-    <Population>3</Population>
-    <BuildTime>60</BuildTime>
     <Resources>
       <food>500</food>
       <metal>500</metal>

binaries/data/mods/public/simulation/templates/template_unit_hero_infantry.xml

@@ -1,7 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Entity parent="template_unit_hero">
   <Cost>
-    <BuildTime>40</BuildTime>
     <Resources>
       <food>200</food>
       <wood>200</wood>