Removed old product when subscription is updated
Removed old product when subscription is updated

refs https://github.com/TryGhost/Team/issues/979

This correctly handles updates to subscriptions so that if the product a
subscription is for has changed, we will remove the previous product, if
and only if there is not another subscription which gives access to it.

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost