remove stellar auth (#1262)

docs/build/apps/application-design-considerations.mdx

@@ -34,7 +34,6 @@ Building an application with [multi-signature](../../learn/encyclopedia/security
 
 There are several apps and services that specialize in adding additional security layers to users' accounts. Check them out if you're interested in integrating a third-party key management service:
 
-- [StellarAuth](https://stellarauth.com/)
 - [Ledger](https://www.ledger.com/)
 - [Trezor](https://trezor.io/)
 - [StellarGuard](https://stellarguard.me/)