[System.Web] Remove NET_2_0 from HttpConfigurationSystem
[HttpConfigurationSystem] Proxy to previous system if section is empty

If the new HttpConfigurationSystem was put in place, there was no way to
access the old system for accessing non-System.Web settings, even though
ChangeConfigurationSystem() method was providing it as a return value.

Proxying to previous system as a fallback fixes bxc#11972:
https://bugzilla.xamarin.com/show_bug.cgi?id=11972
[System.Web] Remove unneeded namespace prefix in a test

This prefix was needed to demonstrate that the first access to AppSettings
worked while the second didn't. If we simply access the AppSettings once,
after HttpConfigurationSystem is in place, the test will simply pass if
the fix for bug BXC#11972 is applied, which is what we care about.