Remove old nav code, make the new nav the default

[rosslovas]

Summary

We've been able to try out the new nav for a while with ?newnav appended to the URL, but the old header was still the experience customers saw, as the new nav was missing functionality.

I believe at this point the new nav has everything we need, so this PR attempts to delete all the code related to the old header, and makes the new header the default.

Results

Test it out at: https://stoctodocspr3381.z22.web.core.windows.net/docs

To test the signed-in state:

document.cookie = 'OctopusSignedInUser=' + encodeURIComponent(JSON.stringify({fullName: 'John Doe', email: 'jd@example.com'})) + ';path=/';
window.dispatchEvent(new CustomEvent('octopus:user-changed'));

(revert to the signed-out state with:)

document.cookie = 'OctopusSignedInUser=;path=/;max-age=0';
window.dispatchEvent(new CustomEvent('octopus:user-changed'));

Note that there are two relatively distinct experiences worth testing: the ordinary wide experience, and the mobile-friendly experience that appears when the viewport is less than 1130px wide.

[team-marketing-branch-protections]

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