Remove MagicAuthChallenge interface (#851)

## Description

* This interface is no longer used after all related endpoints were
migrated to returning a User object instead.

## Documentation

Does this require changes to the WorkOS Docs? E.g. the [API
Reference](https://workos.com/docs/reference) or code snippets need
updates.

```
[ ] Yes
```

If yes, link a related docs PR and add a docs maintainer as a reviewer.
Their approval is required.

src/users/interfaces/index.ts

@@ -9,7 +9,6 @@ export * from './create-user-options.interface';
 export * from './delete-user-options.interface';
 export * from './enroll-user-in-mfa-factor.interface';
 export * from './list-users-options.interface';
-export * from './magic-auth-challenge.interface';
 export * from './remove-user-from-organization-options.interface';
 export * from './send-magic-auth-code-options.interface';
 export * from './send-verification-email-options';