Cleanup CSS

Removes outdated or never used vendor prefixes and one duplicate property. Saves around 1.5 KB in file size.

Source for gradients and browser market share is http://caniuse.com/#feat=css-gradients

Regarding khtml I suggest the research from @paulirish at Modernizr/Modernizr#454

IMO the the moz prefix could be dropped for gradients as well, but there are a bit more users affected so I didn't want to dump this in here.