Remove ProxyStubs from appxmanifests

[krschau]

Suggested new feature or improvement

Should be done for both extensions, but core Dev Home stays the same.
Default proxy stubs are included in Microsoft.Windows.Widgets.dll and your appxmanifest should be prepopulated with the registration entries when you build with 1.5.

[beervoley]

Host proxy stubs should still stay in place :)

[krschau]

Host proxy stubs should still stay in place :)

Task updated. Thanks!