#674 PurgeService: Always register if purging, reduce code duplication

- Rename autoPurging to isPurging: we should always register if a purge task is in progress (regardless if autopurge or not) and deny any new requests
- Reduce the same logic being coded multiple times by calling through the methods
- DataSource: remove purgeBanned in favor of purgeRecords, both do exactly the same thing

src/main/java/fr/xephi/authme/datasource/SQLite.java

@@ -452,19 +452,6 @@ public class SQLite implements DataSource {
         return 0;
     }
 
-    @Override
-    public void purgeBanned(Set<String> banned) {
-        String sql = "DELETE FROM " + tableName + " WHERE " + col.NAME + "=?;";
-        try (PreparedStatement pst = con.prepareStatement(sql)) {
-            for (String name : banned) {
-                pst.setString(1, name);
-                pst.executeUpdate();
-            }
-        } catch (SQLException ex) {
-            logSqlException(ex);
-        }
-    }
-
     @Override
     public DataSourceType getType() {
         return DataSourceType.SQLITE;