Data: Avoid unsetting insertUsage preference in block editor migration

[aduth]

Fixes #14580

This pull request seeks to remove logic which would unset the insertUsage preference when migrating preferences to the block-editor module introduced in #13088 . This intends to resolve an issue where downgrading to an earlier version or deactivating the plugin could result in an editor crash due to the missing property.

Testing instructions:

1. (Prerequisite) Update to WordPress v5.1.1
   • In other words, be running the latest version, or at least not WordPress v5.2.0-beta.1
2. Deactivate Gutenberg from the Plugins screen
3. Navigate to Posts > Add New
4. Clear localStorage
   • localStorage.clear() in your Developer Tools Console
5. Reload
6. Click on the writing prompt
   • This initializes the insertUsage preference in core/editor.
7. Activate Gutenberg plugin from the Plugins screen
   • Make sure the plugin is a built version of this branch.
8. Navigate to Posts > Add New
   • You don't need to take any action, just visit. The preference migration takes place automatically.
9. Deactivate Gutenberg from the Plugins screen
10. Navigate to Posts > Add New
11. Click the Inserter
12. Verify that no error occurs

[youknowriad]

Looks like there's an eslint issue, but approving as it would be good to include in the next release and backport to WP 5.2

[gziolo]

@youknowriad - how about 8b796e9? It should fix the issue.

[gziolo]

Well, we can polish later, let's merge it and re-test in master branch :)