rustdoc mobile: move notable traits to return type

These were originally on the left, but were moved to the return type in
c90fb71. The CSS rule for mobile did
not get updated at the time, so updating it now.

Author: jsha

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

@@ -1973,12 +1973,6 @@ in storage.js plus the media query with (min-width: 701px)
 		display: none !important;
 	}
 
-	.notable-traits {
-		position: absolute;
-		left: -22px;
-		top: 24px;
-	}
-
 	#titles > button > div.count {
 		float: left;
 		width: 100%;