Remove accounts popup from download flow (Fix #14246)

[stephaniehobson]

One-line summary

Remove accounts popup from download flow.

Significant changes and points to review

• Remove JS, CSS, and associated HTML
• Remove associated tests
• Remove a few other unused strings that use the "Join Firefox" language

Issue / Bugzilla link

Fix #14246

Testing

1. Using Firefox but don't be signed into a Mozilla account
2. Try to download from /new
3. Get a download (instead of the popup)

• Make sure I didn't break downloads 😬
• There was CSS on the page for a second modal (other-platforms) but I can't find the HTML that goes with that so I removed it but I'm worried I am missing something but I really can't find it.

✅ Integration tests.