Remove StripeAuth everywhere from the API

[joshsmith]

What's in this PR?

Removes StripeAuth everywhere since we're lo longer doing OAuth connection for standalone accounts.

References

Fixes #556

[joshsmith]

This is a little hacky on the implementation of StripeConnectAccountService.create/1, but I figured that we could always work to replace this in #564 when the behavior actually changes, and depending on upstream changes from stripity_stripe this will change anyway. This decision here at least unblocks that without much detriment at present.

[begedin]

I so love red PRs :)