rustdoc: remove unused mobile CSS .rustdoc { padding-top: 0 }

When this rule was added in dd437ee, as
`body { padding-top: 0 }`, the desktop body tag had non-zero top padding.
This padding was removed in 135281e.

This rule no longer overrides a rule in rustdoc's desktop styles, and also
doesn't override the UA stylesheet, since the [HTML standard] has only
margin, not padding, on the page body.

[HTML standard]: https://html.spec.whatwg.org/multipage/rendering.html#the-page

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

@@ -1677,7 +1677,6 @@ in storage.js
 	}
 
 	.rustdoc {
-		padding-top: 0px;
 		/* Sidebar should overlay main content, rather than pushing main content to the right.
 		   Turn off `display: flex` on the body element. */
 		display: block;