Remove redundant call to worldMaps.a(). Addresses BUKKIT-4828

The WorldMapCollection object for SecondaryWorldServers(Nether, End) is
shared with the main world, so saving it again for each SecondaryWorldServer
is redundant.

This commit removes the redundant call by checking if the WorldServer is an
instanceof SecondaryWorldServer before invoking worldMaps.a().

src/main/java/net/minecraft/server/WorldServer.java

@@ -779,8 +779,12 @@ public class WorldServer extends World implements org.bukkit.BlockChangeDelegate
     protected void a() throws ExceptionWorldConflict { // CraftBukkit - added throws
         this.G();
         this.dataManager.saveWorldData(this.worldData, this.server.getPlayerList().q());
+        // CraftBukkit start - save worldMaps once, rather than once per shared world
+        if (!(this instanceof SecondaryWorldServer)) {
             this.worldMaps.a();
         }
+        // CraftBukkit end
+    }
 
     protected void a(Entity entity) {
         super.a(entity);