Remove deprecated warning for a method that aren't deprecated.

Posting to the /customaudiences endpoint is not deprecated. See docs here: https://developers.facebook.com/docs/reference/ads-api/custom-audience-targeting/v2.2

Also, removed the hmac import and appsecret_proof, since it wasn't being used anywhere. (And, app_secret should not be in the scope of this API anyway, IMO... only one method uses it and it's a graph method, not an Ads API method.)