Incorrect assumption about FCM endpoint

[This code][link] asserts that an auth key (GCM sender ID) is required when a push subscription is sent from FCM. However, this is not true because FCM fully implements VAPID, as can be read [here][link2]: "With VAPID you no longer need to sign up for an account with GCM".

My suggestion therefore is to remove this function clause altogether, as well as [this part](https://github.com/danhper/elixir-web-push-encryption/blob/master/lib/web_push_encryption/push.ex#L69-L70).

[link]: https://github.com/danhper/elixir-web-push-encryption/blob/master/lib/web_push_encryption/push.ex#L33

[link2]: https://developers.google.com/web/updates/2016/07/web-push-interop-wins#introducing_vapid_for_server_identification