remove SnsSubscriptions from oldTopic

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The SnsSubscription function manages to remove old subscriptions in the _current_ topic when searching for subscriptions [here](https://github.com/mapbox/magic-cfn-resources/blob/7571a63189e18e22c5b8369ab1989e8d8ef43bf0/functions/sns-subscription.js#L105-L110) but it does not remove subscriptions if the topic has changed in a stack update.

For example:

1. stack update subscribes `Endpoint1` to `TopicA`
2. then stack is updated to subscribe `Endpoint1` to `TopicB` instead of `TopicA`

The `delete` method does not search `TopicA` for subscriptions to remove, only `TopicB` to avoid duplicate subscriptions. This means `Endpoint1` will continue to receive messages from `TopicA` until it the subscription is manually deleted by an engineer.

We should update the lambda function to search for subscriptions in the oldTopic and newTopic.

Start in functions/sns-subscription.js around lines 105-110 and trace how the delete method searches for subscriptions during a stack update. Verify how oldTopic and newTopic are provided; done when subscriptions are removed from both topics without creating duplicate subscriptions.

Written by the indexing model from the issue text.