[web] Cleanup: HostNodes

[ditman]

Flutter has a rendering mode where it puts all its contents in a regular DOM element when the ShadowDOM API is not supported.

This was implemented for Stadia, but now that it's sadly not needed, it can maybe be cleaned up.

Proposal

Completely delete lib/web_ui/lib/src/engine/host_node.dart, then fix whatever fails where that type is used. Substitute this by a standard DomShadowRoot type (from dom.dart).

[ditman]

Does this break the ability of Cobalt to render Flutter web apps?

[ditman]

Follow up of: flutter/engine#37738

[github-actions]

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