Jetpack: remove old widget migration code

[sergeymitr]

Proposed changes:

Remove old migration code for legacy Gallery and Image widgets.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

https://github.com/Automattic/vulcan/issues/529
p9dueE-56-p2
p1730913512335599-slack-C05PV073SG3

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

1. Make sure the tests pass.
2. Try using one of the widgets from the Widget module in a post, confirm it renders well.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the update/optimize-widget-migration-hooks branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack update/optimize-widget-migration-hooks
  

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[github-actions]

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[jeherve]

Instead of updating the logic, should we consider removing it entirely? Those migrations were introduced 7 and 8 years ago. Maybe we can stop trying to migrate folks today, and assume that folks who needed to migrated have migrated already?
Given that the 2 core widgets we attempt to migrate to (image and gallery) are now legacy widgets (focus is on blocks instead), I think it makes even less sense to keep the migration to legacy widgets around.