rustdoc: remove unused CSS selector a.source

The link with this class attribute was removed in
4d16de0.

Author: notriddle

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

@@ -201,7 +201,6 @@ h4.code-header {
 h1, h2, h3, h4, h5, h6,
 .sidebar,
 .mobile-topbar,
-a.source,
 .search-input,
 .search-results .result-name,
 .item-left > a,