Cleanup wrapper function when app has done quitting

The app may still do something when quitting, we need to make sure the
wrapper function is still there at that time.

atom/browser/api/lib/web-contents.coffee

@@ -112,7 +112,6 @@ wrapWebContents = (webContents) ->
     @_printToPDF printingSetting, callback
 
 binding._setWrapWebContents wrapWebContents
-process.once 'exit', binding._clearWrapWebContents
 
 module.exports.create = (options={}) ->
   binding.create(options)