rustdoc: remove redundant mobile .source > .sidebar CSS

When the source sidebar and standard sidebar had most of their code merged in
07e3f99, the properties `z-index: 11`,
`margin: 0`, and `position: fixed` were already being set on the `.sidebar`
class, so no need to repeat them.

src/librustdoc/html/static/css/rustdoc.css

@@ -1770,9 +1770,6 @@ in storage.js plus the media query with (min-width: 701px)
 	}
 
 	.rustdoc.source > .sidebar {
-		position: fixed;
-		margin: 0;
-		z-index: 11;
 		width: 0;
 	}