rustdoc: remove no-op CSS .source pre { overflow: auto }

Since source pages use the `example-wrap` wrapper, this rule became
redundant because there is already an `overflow-x: auto` rule.

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

@@ -523,7 +523,6 @@ ul.block, .block li {
 }
 
 .source .content pre.rust {
-	overflow: auto;
 	padding-left: 0;
 }