Bug 1894064: remove migration code

[rosahbruno]

The only documentation that we had of this was in the CHANGELOG, so there are no accompanying doc changes.

Why this existed

We are removing this migration code since it is no longer needed. The reason for this code was to help move client_ids and first_run_dates over from our previous storage solution from when Glean.js was asynchronous. Newer versions of Glean.js use localStorage which allows synchronous read/writes. Glean.js used to use IndexedDB which forced asynchronous read/writes.

Synchronous is necessary so that we can ensure read/writes execute before pages are unloaded. If we start to write to storage async and the page unloads, that process never finishes and we incur data loss.

Why it can go away

The only product that used Glean.js in production that was migrating to our new storage solution was MDN. They have been the only ever consumer of this feature. We have had the migration code running for ~9 months in MDN, so the majority of users would have migrated over their data by now.

We are removing it because of an error we are seeing when submitting the first ping for any user before they have migrated. A ping is submitted missing both the client_id and the first_run_date since they get lost in a race condition. This is causing a lot of schema ingestion errors.

We've run the migration code for long enough and now it is safe to turn it off. This will also stop the large amount of first_run_date errors that we have been seeing.

Pull Request checklist

• Quality: Make sure this PR builds and runs cleanly.
  • Inside the glean/ folder, run:
    • npm run test Runs all tests
    • npm run lint Runs all linters
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry to CHANGELOG.md or an explanation of why it does not need one
• Documentation: This PR includes documentation changes, an explanation of why it does not need that or a follow-up bug has been filed to do that work
  • Documentation should be added to The Glean Book when making changes to Glean's user facing API
    • When changes to the Glean Book are necessary, link to the corresponding PR on the mozilla/glean repository
  • Documentation should be added to the Glean.js developer documentation when making internal code changes