removed ARC2_Class::queryDB (breaks backward compatibility)

It worked only with mysqli, but since 2.4 we use an adapter approach.
therefore tight coupling with mysqli is no longer desired.

ARC2_Class.php

@@ -503,44 +503,6 @@ public function getDBObjectFromARC2Class($con = null)
         return $this->db_object;
     }
 
-    /**
-     * Dont use this function to directly query the database. It currently works only with mysqli DB adapter.
-     *
-     * @param string $sql SQL query
-     * @param mysqli $con Connection
-     * @param int    $log_errors 1 if you want to log errors. Default is 0
-     *
-     * @return mysqli Result
-     *
-     * @deprecated since 2.4.0
-     */
-    public function queryDB($sql, $con, $log_errors = 0)
-    {
-        $t1 = ARC2::mtime();
-
-        // create connection using an adapter, if not available yet
-        $this->getDBObjectFromARC2Class($con);
-
-        $r = $this->db_object->mysqliQuery($sql);
-
-        // TODO check if this is ever called. it seems not and therefore could be removed.
-        if (0) {
-            $t2 = ARC2::mtime() - $t1;
-            $call_obj = $this;
-            $call_path = '';
-            while ($call_obj) {
-                $call_path = get_class($call_obj) . ' / ' . $call_path;
-                $call_obj = isset($call_obj->caller) ? $call_obj->caller : false;
-            }
-            echo "\n" . $call_path . " needed " . $t2 . ' secs for ' . str_replace("\n" , ' ', $sql);;
-        }
-
-        if ($log_errors && !empty($this->db_object->getErrorMessage())) {
-            $this->addError($this->db_object->getErrorMessage());
-        }
-        return $r;
-    }
-
   /**
    * Shortcut method to create an RDF/XML backup dump from an RDF Store object.
    */