Unset t:cwd on active tab page

This stops `t:cwd` shadowing the current working directory while possibly getting out of sync with it.


Usage case example: setting `guitablabel` to display the tail of the current working directory – this needs `t:cwd` to be set for inactive tabs to display a correct value, but the active tab gets out of sync when we `cd` around and `t:cwd` is not updated to reflect that. Admittedly a corner case, but it can be resolved without any side effects by this PR AFAIK.