Remove amp-ad-exit logic from Story Ads

[calebcordry]

In #24044 I will be introducing some logic into amp-story-auto-ads that will look for an amp-ad-exit config and if it exists, try to extract an url for the CTA click.

This was introduced as a temporary solution until we have first class support from our partner ad networks.

To delete this code we will need ad networks to support writing click tracking urls directly into the meta tag that story ads try to read
<meta name="amp-cta-url" content="<CLICK_TRACKING_URL">

cc/ @ampproject/wg-ads

[stale]

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