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)