rustdoc: merge separate .item-info CSS

Rough timeline:

* The longer `.content .item-info` selector originated in
  110e727. No reason seems to be given in
  the PR why it needed the `.content` part, but it was probably added because
  of <https://github.com/rust-lang/rust/blob/110e7270ab7b0700ce714b8b1c7e509195dea2c4/src/librustdoc/html/static/rustdoc.css#L476-L478>.
  That selector with the margin-bottom was removed when CSS containment
  was added in 8846c08.
* `.stability` was renamed `.item-info` in
  caf6c57.
* The selector without the `.content` was added in
  d48a39a.

Author: notriddle

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

@@ -694,16 +694,13 @@ pre, .rustdoc.source .example-wrap {
 
 .item-info {
 	display: block;
+	margin-left: 24px;
 }
 
-.content .item-info code {
+.item-info code {
 	font-size: 0.875rem;
 }
 
-.content .item-info {
-	margin-left: 24px;
-}
-
 #main-content > .item-info {
 	margin-top: 0;
 	margin-left: 0;
@@ -1948,7 +1945,7 @@ in storage.js plus the media query with (min-width: 701px)
 	}
 
 	/* Align summary-nested and unnested item-info gizmos. */
-	.content .impl-items > .item-info {
+	.impl-items > .item-info {
 		margin-left: 34px;
 	}
 }