Remove linear-gradient from single chosen input background

Fixes #1728 

Originally I was going to add a background-position for the linear-gradient, so that Chrome wouldn't override it with the -20px it was getting from the sprite (causing the weird line). However, after realizing that 1) there is no linear-gradient on retina because of the `!important`, and 2) the linear-gradient is hardly visible anyway, I think we should just remove it.