Remove duplication of code for Stripe integration

[tobinmori]

Summary After https://github.com/mdn/kuma/pull/6756 lands, we'll have 2 distinctly different implementations of the Stripe form. That's a bad thing. Let's get rid of the implementation that sits on the Edit Profile page.

We also need to find a new home for the link, for existing subscribers, to go to the Subscription Management page where they can cancel it.

Much of this comes from this discussion.

Rationale Duplication is bad. It doubles the maintenance burden and the risk factors of things that can go wrong and get out-of-sync in terms of functionality. Tech debt is just as important as new-feature work.

Workaround Let it be repeated and swallow the risk + tech debt and pay the price another day instead of now.