Minor cleanup in ArenaMaster.

Removes unused imports and fields, trailing whitespace.

src/main/java/com/garbagemule/MobArena/ArenaMasterImpl.java

@@ -25,7 +25,6 @@ import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
-import java.util.Objects;
 import java.util.Set;
 import java.util.function.Consumer;
 import java.util.stream.Collectors;
@@ -36,7 +35,6 @@ public class ArenaMasterImpl implements ArenaMaster
 
     private List<Arena> arenas;
     private Map<Player, Arena> arenaMap;
-    private Arena selectedArena;
 
     private Map<String, ArenaClass> classes;
 
@@ -301,7 +299,6 @@ public class ArenaMasterImpl implements ArenaMaster
         ConfigurationSection section = makeSection(plugin.getConfig(), "classes");
         ConfigUtils.addIfEmpty(plugin, "classes.yml", section);
 
-
         // Establish the map.
         classes = new HashMap<>();
         Set<String> classNames = section.getKeys(false);