Respect theme features suppressed by theme .info file

I found an old @todo in the module file... Apparently it occurred to me once before that logo and favicon could be suppressed by commenting them out in the .info file. When this happens, their respectivwe fieldset is not built for the system_theme_settings form.

I've just tested the behaviour and discovered that I end up with three radio buttons for a suppressed favicon. Need to fix it, this is a release blocker for a stable 6.x-1.0.