Hover observables are never unsubscribed

Created by: sqs

Repro:

Create a hover provider in an extension like this:

    sourcegraph.languages.registerHoverProvider(['*'], {
        provideHover: () =>
            interval(100).pipe(
                map(n => ({
                    contents: { kind: sourcegraph.MarkupKind.Markdown, value: `⏱ ️${n}` },
                }))
            ),
    })

1. Enable the extension.
2. Turn on extension host communication trace logging by running localStorage.debug=1;location.reload() and then toggling it on in the Ext menu in the bottom right.
3. Hover over it in Sourcegraph. You will see the hover change frequently.
4. Now hover away from it.

Notice that the JS devtools console shows that it is still sending messages with new hover contents. It should have been unsubscribed.

This is a resource leak and could make things slow. Not high priority since it's unlikely to cause many actual problems, but good to fix anyway.