rustdoc: remove redundant mobile-sized .source nav:not(.sidebar).sub

It's redundant because there's already a selector `.source nav.sub` with
exactly the same margin-left at line 796.

This selector was added in 1e98fb1, along
with an identical desktop selector, but that desktop selector was removed in
6a5f8b1 as part of a larger simplification.

Author: notriddle

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

@@ -1887,10 +1887,6 @@ in storage.js plus the media query with (min-width: 701px)
 		background-color: var(--sidebar-background-color);
 	}
 
-	.source nav:not(.sidebar).sub {
-		margin-left: 32px;
-	}
-
 	.content {
 		margin-left: 0px;
 	}