rustdoc: remove outdated CSS .sub-variant > div > .item-info

This CSS still matches sometimes, as you can see in
<https://doc.rust-lang.org/1.63.0/std/collections/enum.TryReserveErrorKind.html#variant.AllocError.fields>,
but since nothing else is setting `margin-top`, putting it back to `initial`
does nothing.

This selector was added in 2fd378b (but it
was called `.stability` instead of `.item-info` at the time), probably as an
override for the selector immediately above it that sets a negative margin.
That negative margin was removed in 593d6d1.

Author: notriddle

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

@@ -770,10 +770,6 @@ pre, .rustdoc.source .example-wrap {
 	margin-left: 24px;
 }
 
-.sub-variant > div > .item-info {
-	margin-top: initial;
-}
-
 .content .impl-items .docblock, .content .impl-items .item-info {
 	margin-bottom: .6em;
 }