- removes `about:firefoxview-next` route
- `about:firefoxview` now points to `firefoxview-next.html` (until we rename)
- remove pref for `browser.tabs.firefox-view-next`
- preserve pref for `browser.tabs.firefox-view-newIcon` as part of firefoxViewNext feature manifest (until experiment is over)
- whitelist unreferenced files in `browser_all_files_referenced.js` as they will be removed in child Bug 1869833
Differential Revision: https://phabricator.services.mozilla.com/D196093