rustdoc: remove no-op mobile CSS #source-sidebar { z-index: 11 }

This rule became redundant in 07e3f998b1.
When `#source-sidebar` became nested below `.sidebar`, it went from being
`position: fixed` to `position: static`, and according to MDN's [z-index]
documentation, this means it has no effect.

[z-index]: https://developer.mozilla.org/en-US/docs/Web/CSS/z-index

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

@@ -1922,10 +1922,6 @@ in storage.js plus the media query with (min-width: 701px)
 		border-bottom: 1px solid;
 	}
 
-	#source-sidebar {
-		z-index: 11;
-	}
-
 	#main-content > .line-numbers {
 		margin-top: 0;
 	}