rustdoc: remove redundant CSS .source .content { overflow: visible }

When added in 7669f04fb0 / #16066, the page
itself was set to scroll. Now it's set so that the `example-wrap` is
scrolling inside the page, so the overflow setting for the content is
irrelevant.

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

@@ -355,10 +355,6 @@ img {
 	max-width: 100%;
 }
 
-.source .content {
-	overflow: visible;
-}
-
 .sub-logo-container, .logo-container {
 	/* zero text boxes so that computed line height = image height exactly */
 	line-height: 0;