Remove the feature flag from our pkgdef, and legacy editor check

[davidwengier]

For now, we should leave the property on LanguageServerFeatureOptions, otherwise all of the old LSP server tests would fail, and we don't want to necessarily delete all of that code yet, but we can hardcode to true in VisualStudioLanguageServerFeatureOptions.

We should also remove WellKnownSettingNames.UseLegacyASPNETCoreEditor, and we can hardcode the output of the ComputeUseLegacyEditorAsync method.

We could also consider simplifying the content type selection as a whole, and remove the LspFeatureEditorDetector. Additionally we could consider not exporting the old language server and IDynamicFileProvider impl, just to simplify initialization, but that could potentially be a bit too viral.