Handle unsubscribing on legacy active repos

Repos that were enabled before subscription changes won't have a subscription
record. We should allow them to deactive/unsubscribe.

Related error:
https://app.getsentry.com/app20126465herokucom/production/group/28946247/
Like we talked about yesterday, I don't think this is necessary since there is logic in the JavaScript that avoids unsubscribing an active repo without a subscription.