Remove unused assets from torbutton (preferences-mobile.css)

We still have a pair of CSS files and other assets that we should have removed a while ago (I don't remember the exact issue number, now).

toolkit/torbutton/chrome/skin/preferences-mobile.css
toolkit/torbutton/chrome/skin/preferences.css (to double check)
toolkit/torbutton/chrome/skin/tor.png
toolkit/torbutton/chrome/skin/torbrowser_mobile_logo.png (referenced in aboutTor.xhtml, and hidden by aboutTor.css whenever we're not in mobile mode, which doesn't happen anymore, at the moment)

Edited Feb 02, 2023 by Pier Angelo Vendrame

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information