Remove __destructor from MysqliDb

Allow garbage collection to close mysqli instance connection, instead of closing it in the destructor.

MysqliDb.php

@@ -1848,23 +1848,6 @@ class MysqliDb
         return $stmt;
     }
 
-    /**
-     * Close connection
-     * 
-     * @return void
-     */
-    public function __destruct()
-    {
-        if ($this->isSubQuery) {
-            return;
-        }
-
-        if ($this->_mysqli) {
-            $this->_mysqli->close();
-            $this->_mysqli = null;
-        }
-    }
-
     /**
      * Referenced data array is required by mysqli since PHP 5.3+
      *