Removed extra $session_lifetime declaration

Hey, I removed an extra declaration for `$session_lifetime` property, probably a line cloned by mistake in the code. Thanks a lot for your work on this package, I was just about to implement something awfully similar, saved me some time; I'll be sure to ping you whether I'll find anything to improve / repair.

And uhm.. sorry for bothering you with such a lame, short pull request :-)