Remove calls to the old navigation bar.

It is likely that during the implementation & testing of the new navigation bar within the context header that we already know exactly what to replace and where to enact the required changes.

We need to remove the existing navigation bar & tertiary navigation from the context header and replace it with our new navigation bar.

The expectation with this issue is that the navigation bar is already matching the designs & accessible.

Acceptance criteria

• The new context header matches the designs
• theme_boost is the only theme that receives the new navigation bar functionality
• Behat & PHPUnit tests continue to pass across platforms & themes