Decommission the Mozilla Account fallback WNP

[craigcook]

Description

Some time ago we were heavily promoting Mozilla Accounts (nee Firefox Accounts) and made a fallback template for the Firefox what's new page that urges users who are not already signed into Firefox to create an account (users who are signed in see a promotion for Firefox for mobile).

We're not focusing our marketing strategy on accounts as much these days, and that fallback page is only shown to users updating to older versions of Firefox so it's something of an edge case already. So we've decided to just remove that index-account.html template and use a single fallback that only promotes mobile. This will clean up some tech debt and help us simplify the WNP view logic to boot.

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Success Criteria

• Remove the firefox/whatsnew/index-account.html template and associated assets.
• Use the standard index.html template as the sole fallback for WNPs.
• Remove or update any associated tests.