Remove tab highlighting

These features are extracted into 
https://github.com/tabkit/tab-highlighter

- Highlight Current Tab in Black
- Make Current Tab's Text Bold
- Highlight Unread Tabs in Yellow
- Highlight Protected Tabs in Green (Protected Tabs feature requires TabKit 2nd Edition)

That addon is reviewed
There is no point keeping duplicated code here
And the extraction is intended to remove code from this giant from the start