[ci skip] Remove NS usage in CI_Session

It was accidental
diff --git a/system/libraries/Session/Session.php b/system/libraries/Session/Session.php
index 54d31ee..bb457c6 100644
--- a/system/libraries/Session/Session.php
+++ b/system/libraries/Session/Session.php
@@ -227,7 +227,7 @@
 
 			if ( ! class_exists('CI_'.$class, FALSE) && ! class_exists($class, FALSE))
 			{
-				throw new \UnexpectedValueException("Session: Configured driver '".$driver."' was not found. Aborting.");
+				throw new UnexpectedValueException("Session: Configured driver '".$driver."' was not found. Aborting.");
 			}
 		}