Remove all players from boss bars when clearing the MonsterManager.

When the arena ends, the MonsterManager is cleared, which means that all references to health bars are lost. Before this, we remove all players from any boss bars to make sure that the boss bars don't stick to players upon dying.

Fixes #473

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

@@ -1,5 +1,6 @@
 package com.garbagemule.MobArena;
 
+import com.garbagemule.MobArena.healthbar.HealthBar;
 import com.garbagemule.MobArena.waves.MABoss;
 import org.bukkit.entity.Entity;
 import org.bukkit.entity.LivingEntity;
@@ -12,6 +13,7 @@ import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Set;
 
 public class MonsterManager
@@ -43,6 +45,11 @@ public class MonsterManager
     }
     
     public void clear() {
+        bosses.values().stream()
+            .map(MABoss::getHealthBar)
+            .filter(Objects::nonNull)
+            .forEach(HealthBar::removeAll);
+
         removeAll(monsters);
         removeAll(sheep);
         removeAll(golems);