client: Remove unused `cached_user_by_github_login` method from `UserStore` (#60103)

This PR removes the `cached_user_by_github_login` method from the
`UserStore`, as it was unused.

Release Notes:

- N/A
380a5c79b6Marshall Bowers committed on 6/29/2026, 3:38:19 PM· committed by GitHubparent7c3160b
1 file changedLine totals unavailable