Global header remains sticky even though the shared component has been unstuck

[adamwoodnz]

After WordPress/wporg-mu-plugins#466 is merged, the global header should not be sticky anymore, and only the local nav should be.

Steps to reproduce:

1. Set your mu-plugins to Global Header: Try unsticking the global header, making local navigation sticky wporg-mu-plugins#466
2. Scroll the home page
3. Expect the global header to not be sticky, but it still is

Extra work will be required to make the local nav sticky, separate from the global header. https://wordpress.org/download/releases/ has almost the same header style.