Remove dirty session tracking (it's too fragile)

I never liked this code and the additional complexity it introduced, so
lets try without for a while and see if anyone complains. If I really want
to I can always get the code back from version control.

Pull request #37: https://github.com/xolox/vim-session/pull/37
Issue #38: https://github.com/xolox/vim-session/issues/38