Remove CI_DB_pdo_driver::

Improving on PR #2265, the property is inherited with the same value and doesn't need to be set.
diff --git a/system/database/drivers/pdo/pdo_driver.php b/system/database/drivers/pdo/pdo_driver.php
index ffbafea..fa89661 100644
--- a/system/database/drivers/pdo/pdo_driver.php
+++ b/system/database/drivers/pdo/pdo_driver.php
@@ -49,13 +49,6 @@
 	public $dbdriver = 'pdo';
 
 	/**
-	 * Transaction enabled flag
-	 *
-	 * @var	bool
-	 */
-	public $trans_enabled = TRUE;
-
-	/**
 	 * PDO Options
 	 *
 	 * @var	array