Downloaded updates are pruned by subsequent update checks

[joshaber]

This was introduced by #171 🙈:

1. Check for updates.
2. Prune existing downloads.
3. Download an update.
4. Check for updates.
5. Prune existing downloads.
6. Conditional GET means we don't re-download.
7. Now we don't have an update downloaded anymore.

[joshaber]

Interestingly, this means we've always had the possibility that:

1. Check for updates.
2. Prune existing downloads.
3. Download an update.
4. Check for updates.
5. Prune existing downloads.
6. Quit the app.
7. We try to install an update that's no longer downloaded.