Apply minor changes to PurgeService

- Remove various imports / unused fields
- Make CacheDataSource call source for purging on DB
- Minor: SQLite - place creation of PreparedStatement outside of loop
- Make specific purge actions called from task package-private (clearer from the outside which methods can be called from the outside)

src/main/java/fr/xephi/authme/permission/handlers/GroupManagerHandler.java

@@ -3,7 +3,6 @@ package fr.xephi.authme.permission.handlers;
 import fr.xephi.authme.permission.PermissionNode;
 import fr.xephi.authme.permission.PermissionsSystemType;
 import org.anjocaido.groupmanager.GroupManager;
-import org.anjocaido.groupmanager.data.User;
 import org.anjocaido.groupmanager.permissions.AnjoPermissionsHandler;
 import org.bukkit.Bukkit;
 import org.bukkit.entity.Player;