adjust node.js web.config template for S21

**NOTE** This pull request should be merged only after Antares S21 bits have been updated with changes to the WebPI feed that install new iisnode bits. That should happen during the week of 2/18. 

The purpose of this pull request is to remove customizations of iisnode configuration from the web.config file that Kudu drops for node.js apps deployed without one. Similar customizations (but with different defaults) will now be applied machine-wide during installation of iisnode using the WebPI feed. This mechanism ensures that node.js apps deployed through Kudu and WebDeploy will get the same defaults.