Finalized some fields, removed redundant code, various other fixes

src/main/java/fr/xephi/authme/cache/auth/PlayerAuth.java

@@ -16,7 +16,7 @@ public class PlayerAuth {
     private double z;
     private String world;
     private String salt;
-    private int groupId;
+    private final int groupId;
     private String email;
     private String realName;
 

src/main/java/fr/xephi/authme/cache/auth/PlayerCache.java

@@ -7,7 +7,7 @@ import java.util.concurrent.ConcurrentHashMap;
 public class PlayerCache {
 
     private volatile static PlayerCache singleton;
-    private ConcurrentHashMap<String, PlayerAuth> cache;
+    private final ConcurrentHashMap<String, PlayerAuth> cache;
 
     private PlayerCache() {
         cache = new ConcurrentHashMap<>();