rustdoc: remove redundant CSS .out-of-band > span.since { position }

At the time this CSS was added, it was just `span.since`, because the
version info could be rendered in two different ways:

 1. `<div class='since'>` was used for associated items like methods. It
    was absolutely positioned, and the selector in rustdoc.css that
    targetted it was just `.since`.

    a5a2f2b951/src/librustdoc/html/static/rustdoc.css (L522-L529)

 2. `<span class='since'>` was introduced in
    a5a2f2b951 for page-global version info,
    so that it could be laid out alongside the `[-]`/`[+]` button. This CSS
    rule was added to override the absolute position introduced in (1).

    a5a2f2b951/src/librustdoc/html/static/rustdoc.css (L637-L641)

The selector was changed in 8fc6e420d1 so
that everything could use a `<span>` tag, but the dichotomy of the
absolutely-positioned version info for associated items and the static
positioned item version info remained.

The absolutely positioned `.since` was changed to one nested below a
`<div class="rightside">` container in
5de1391b88, so the version information is now
always statically-positioned, and, as described in the commit message,
"their DOM representation is consistent."

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

@@ -1274,7 +1274,6 @@ a.test-arrow:hover {
 }
 
 .out-of-band > span.since {
-	position: initial;
 	font-size: 1.25rem;
 }