Do not merget yet: Remove the feature flag for Newsletter Categories settings

[JuanLucha]

☢️☢️☢️ Don't merge this PR until all the translations are ready ☢️☢️☢️

Fixes https://github.com/Automattic/wp-calypso/issues/88035

Proposed changes:

Removing the feature flag for Newsletter settings in JP

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?
• [ ] Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

No need

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

No

Testing instructions:

• Apply this PR and run your JT site.
• Go to /wp-admin/admin.php?page=jetpack#/newsletter and check that you can see the newsletter settings.

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• 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 remove/feature-flags branch.

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[yansern]

Maybe removing enable-email-settings can be done here too. Context: https://github.com/Automattic/jetpack/pull/36290/files

This line: https://github.com/Automattic/jetpack/pull/36290/files#diff-7122c5abad70a93ba11fd2d8cc19fb9db328aa3da2614f1b8f3c122369ea60adR16