Revert "#3073 (feature/session): Fix a GC issue with the files driver"

This reverts commit c5519ce166943b744bbea75a87831132aced4247.
diff --git a/system/libraries/Session/drivers/Session_files_driver.php b/system/libraries/Session/drivers/Session_files_driver.php
index ff1553f..a4f1b9f 100644
--- a/system/libraries/Session/drivers/Session_files_driver.php
+++ b/system/libraries/Session/drivers/Session_files_driver.php
@@ -261,7 +261,7 @@
 			// If the filename doesn't match this pattern, it's either not a session file or is not ours
 			if ( ! preg_match('/(?:[0-9a-f]{32})?[0-9a-f]{40}$/i', $file)
 				OR ! is_file($this->_config['save_path'].DIRECTORY_SEPARATOR.$file)
-				OR ($mtime = fileatime($this->_config['save_path'].DIRECTORY_SEPARATOR.$file)) === FALSE
+				OR ($mtime = filemtime($this->_config['save_path'].DIRECTORY_SEPARATOR.$file)) === FALSE
 				OR $mtime > $ts)
 			{
 				continue;