Remove the Favorite API & block, use shared block

[ryelle]

This removes the favorite block & API in favor of the new shared favorite button. The API endpoints are not necessary with the shared block's endpoint + callback process, so I've removed them here.

See WordPress/wporg-mu-plugins#617

To test

• Use the branch here New block: Favorite button wporg-mu-plugins#635
• Make sure to build both projects
• View the theme directory
• There are no favorite buttons on the grid view
• View a single theme
• If you're logged in, you should see a button in the theme header
• You should be able to click or trigger with a keyboard to favorite/unfavorite items, and the button should update with the interaction
• Reload the page or view /browse/favorites to ensure the favorite saved correctly
• If you are not logged in, there is no favorite button