Remove _serialize() and _unserialize() methods

Since removing the unneeded manual escaping code, there is no-longer any reason
to have the serialization functions abstracted. This also allows us to only
suppress errors when unserializing cookie data, and only trim when we are
unserializing database data (see commit 6b8312).
diff --git a/system/libraries/Session/drivers/Session_cookie.php b/system/libraries/Session/drivers/Session_cookie.php
index 124e009..dc75d8e 100644
--- a/system/libraries/Session/drivers/Session_cookie.php
+++ b/system/libraries/Session/drivers/Session_cookie.php
@@ -397,7 +397,7 @@
 		}
 
 		// Unserialize the session array
-		$session = $this->_unserialize($session);
+		$session = @unserialize($session);
 
 		// Is the session data we unserialized an array with the correct format?
 		if ( ! is_array($session) OR ! isset($session['session_id'], $session['ip_address'], $session['user_agent'], $session['last_activity']))
@@ -472,7 +472,7 @@
 			$row = $query->row();
 			if ( ! empty($row->user_data))
 			{
-				$custom_data = $this->_unserialize($row->user_data);
+				$custom_data = unserialize(trim($row->user_data));
 
 				if (is_array($custom_data))
 				{
@@ -608,7 +608,7 @@
 			if ( ! empty($userdata))
 			{
 				// Serialize the custom data array so we can store it
-				$set['user_data'] = $this->_serialize($userdata);
+				$set['user_data'] = serialize($userdata);
 			}
 
 			// Reset query builder values.
@@ -696,7 +696,7 @@
 				: $this->userdata;
 
 		// Serialize the userdata for the cookie
-		$cookie_data = $this->_serialize($cookie_data);
+		$cookie_data = serialize($cookie_data);
 
 		if ($this->sess_encrypt_cookie === TRUE)
 		{
@@ -737,36 +737,6 @@
 	// ------------------------------------------------------------------------
 
 	/**
-	 * Serialize an array
-	 *
-	 * This function serializes an array
-	 *
-	 * @param	mixed	Data to serialize
-	 * @return	string	Serialized data
-	 */
-	protected function _serialize($data)
-	{
-		return serialize($data);
-	}
-
-	// ------------------------------------------------------------------------
-
-	/**
-	 * Unserialize
-	 *
-	 * This function unserializes a data string
-	 *
-	 * @param	mixed	Data to unserialize
-	 * @return	mixed	Unserialized data
-	 */
-	protected function _unserialize($data)
-	{
-		return @unserialize(trim($data));
-	}
-
-	// ------------------------------------------------------------------------
-
-	/**
 	 * Garbage collection
 	 *
 	 * This deletes expired session rows from database