rustdoc: remove unused CSS #implementations-list > h3 > span.in-band

This was added in 51f26acaea to help with the
display of an `<h3>` tag that has a `<span class='in-band'>` inside.

The way implementation lists were rendered was changed in
34bd2b845b to have `<code class='in-band'>`,
making this CSS unused.

Then it was turned into a `<div>` in 9077d540da
without issue.

Finally, the header itself acquired the `in-band` class in
76a3b609d0.

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

@@ -1528,10 +1528,6 @@ kbd {
 	cursor: default;
 }
 
-#implementations-list > h3 > span.in-band {
-	width: 100%;
-}
-
 #main-content > ul {
 	padding-left: 10px;
 }