rustdoc: remove unused CSS `#search { position: relative }`
This was added in 611d0e6cce, to allow its child `#results` element to be absolutely positioned inside it. The child stopped being absolute in 8c0469552e.
To keep the layout looking the same, the links need to not have `width: 100%` any more, relying instead on the box naturally growing to fit because it has `display: block`.