This PR removes writes to the `github_user_id` and `github_login`
columns on the `users` table.

These writes were only being done in tests, as the production codepaths
that referenced these columns have already been removed.

Depends on https://github.com/zed-industries/cloud/pull/2743 to make the
`github_user_id` column nullable.

The corresponding database schema migration has been created in the
Cloud repo and applied to the production database.

Closes CLO-831.

Release Notes:

- N/A