Remove razor support for HTML

[octref]

https://github.com/aspnet/Razor.VSCode handles Razor files much better. We should remove its support from our built-in HTML language features.

@aeschli What do you think?

[aeschli]

I agree! @egamma Ok for you?

But we should add a extension recommendation for to help razor users to move to the new extension.

https://github.com/Microsoft/vscode-distro/blob/master/quality/stable/product.json

[octref]

Verification: Verify that cshtml files now prompt C# extension as recommendation

[alexr00]

Should there be a notification, or just C# listed in the Recommended Extensions view? I don't see a prompt and the language in the bottom right corner is still razor.

[octref]

I see, I should have made that a strong recommendation...