Remove duplicate js-hide style info from signup.css

The js-hide support in signup.css is now duplicate with core's system.css, thanks to #144919: use jQuery to hide user picture settings when disabled. It can be safely removed from signup.css (and we can avoid loading signup.css in most of the places we're currently loading it).

However, a patch for this is going to conflict with #335563: allow admins to specify weight for the signup info on nodes and I'm not sure if I want to commit that yet or not... ;) So, I'm going to wait to work on a patch for this until the fate of #335563 is known. ;)