Remove unused data store

[noahtallen]

See #74399 for more info

Proposed Changes

As I was working on migrating some data stores to createReduxStore, I found that this data store (automattic/whats-new) is never used anywhere. It appears we migrated to using inline state instead.

Testing Instructions

• sync ETK to your sandbox and sandbox a test site.
• When loading the editor verify that the "what's new" feature works (it's available in the three-dot dropdown menu in the top-right.)

[matticbot]

This PR modifies the release build for editing-toolkit

To test your changes on WordPress.com, run install-plugin.sh editing-toolkit remove-unused-whatsnew-store on your sandbox.

To deploy your changes after merging, see the documentation: PCYsg-mMA-p2

[matticbot]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-60002

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-60002&env=jetpack

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-59b5ca96b42eca6e93506548293d294e12fa2f83

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-59b5ca96b42eca6e93506548293d294e12fa2f83&env=jetpack

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[tyxla]

LGTM 👍

FWIW the @automattic/whats-new package is responsible for that functionality, and it now uses react query so even if that store was considered to be used, I don't think it will be.

🚀