Remove unused stored_location analytics parameter

[aduth]

🛠 Summary of changes

Removes the stored_location analytics property from the 'Sign in page visited' and 'Email and Password Authentication' analytics events.

This references a session value user_return_to that is not assigned, last referenced removed in #5413.

Tangentially-related Slack discussion: https://gsa-tts.slack.com/archives/C0NGESUN5/p1706906370684589

Why?

• Reduce logging payload size for frequent analytics events
• Avoid misleading developers to imply we track return location for sign-in, since we don't (LG-3737)

📜 Testing Plan

1. In a separate terminal process, run make watch_events
2. Go to http://localhost:3000
3. Observe logged event for "Sign in page visited" does not include stored_location
4. Sign in
5. Observe logged event for "Email and password authentication" does not include stored_location