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/datasource/CacheDataSource.java

@@ -145,6 +145,7 @@ public class CacheDataSource implements DataSource {
 
     @Override
     public void purgeRecords(Set<String> toPurge) {
+        source.purgeRecords(toPurge);
         for (String name : toPurge) {
             cachedAuths.invalidate(name);
         }