Remove vaadin/vaadin-core and vaadin/vaadin in Vaadin 26

[web-padawan]

Describe your motivation

The following repos are still published to npm via template-vaadin-package.json and template-vaadin-core-package.json in Vaadin 25:

• https://github.com/vaadin/vaadin-core
• https://github.com/vaadin/vaadin

We don't actively update their content e.g. imports of components in vaadin-core.js and vaadin.js
Also, these repos should not be used by Flow anymore in recent Vaadin versions.

Describe the solution you'd like

Stop publishing @vaadin/vaadin-core and @vaadin/vaadin to npm starting from V26 (breaking change).