Remove the window.unload event in the KPI event stream.

If we do not need the event for KPI processing, this allows us to avoid making changes to WinChan - see issue #3861

issue #3794
Merge pull request #3864 from mozilla/issue-3794-ditch-window-close

Thanks for the feedback @6a68 and @kparlante - I far prefer the "reduce the amount of code" approach here to the "add more brittle code" approach in the other PR. 

@6a68 gave the r+, @kparlante gave the "approach is good", merging. 

fixes #3794 by removing the offending event.