rustdoc: remove unused CSS .content .item-list

When these rules were added in 4fd061c426
(yeah, that's the very first commit of rustdoc_ng), `.item-list` was a
`<ul>`, and this would override the default style for that tag.

In c1b1d6804b, it was changed to use a
`<div>` tag, so these rules are both no-ops.

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

@@ -690,11 +690,6 @@ pre, .rustdoc.source .example-wrap {
 	border: 1px solid var(--border-color);
 }
 
-.content .item-list {
-	list-style-type: none;
-	padding: 0;
-}
-
 /* Shift "where ..." part of method or fn definition down a line */
 .content .method .where,
 .content .fn .where,