rustdoc: remove no-op CSS .code-header { display: block }

Since 76a3b60 converted code headers to
real headers, `display: block` is now the default.

Author: notriddle

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

@@ -654,10 +654,6 @@ pre.example-line-numbers {
 	font-weight: normal;
 }
 
-.method > .code-header, .trait-impl > .code-header {
-	display: block;
-}
-
 .in-band {
 	flex-grow: 1;
 	margin: 0px;