Update tmux config with alternative to `default-path`

The `default-path` option was removed with tmux v1.9a.
Remove commands to sync with X11 clipboard

I never really used it, and OS X is my primary GUI environment these
days.
Keep showing tmux status bar at the bottom of the window

Option was commented out, but I probably won't use it. And now it's in
Git history if I need to find it later.