rustdoc: remove unused CSS #main-content > .since

This rule was added (actually, it was called `#main > .since` back then) with
cdca084 and you can see an example of the
bug it's intended to fix in <https://doc.rust-lang.org/1.9.0/std/fmt/fn.write.html>
by looking at the `1.0.0` version marker.

However, a5a2f2b changed it so that
`<span class="since">` is always placed in an out-of-band wrapper, so it's
never nested directly below `#main` / `#main-content` any more.

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

@@ -216,7 +216,6 @@ details.rustdoc-toggle > summary::before,
 div.impl-items > div:not(.docblock):not(.item-info),
 .content ul.crate a.crate,
 a.srclink,
-#main-content > .since,
 #help-button > button,
 details.rustdoc-toggle.top-doc > summary,
 details.rustdoc-toggle.top-doc > summary::before,