rustdoc: remove unused CSS .content .methods > div

This selector has its roots in these commits:

  * current version:

    `.content .methods > div:not(.notable-traits):not(.method)` from
    9077d540da

  * intermediate version:

    `.content .methods > div:not(.important-traits)` from
    d86621f69e

  * original version:

    `.content .methods > div { margin-left: 40px; }` from
    0a46933c4d

Based on the call stack, where [`class='methods'`] calls `trait_item` and
[`trait_item`] calls [`document`], this div selector was probably intended to
target docblock and stability tags.

In the current version of the code, neither of these can possibly be nested
directly below the `class='methods'` wrapper, because the [current version of
the `trait_item` function] always wraps them in a `<details>` tag if they
exist. The only div tag that can possibly be nested directly below it now is
the one with class `method`, which is explicitly excluded.

[`class='methods'`]: 0a46933c4d/src/librustdoc/html/render.rs (L1811-L1842)

[`trait_item`]: 0a46933c4d/src/librustdoc/html/render.rs (L1807)

[`document`]: 0a46933c4d/src/librustdoc/html/render.rs (L1515-L1523)

[current version of the `trait_item` function]: e7c7aa7288/src/librustdoc/html/render/print_item.rs (L710)

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

@@ -747,11 +747,6 @@ pre, .rustdoc.source .example-wrap {
 	font-size: 0.875rem;
 }
 
-.content .methods > div:not(.notable-traits):not(.method) {
-	margin-left: 40px;
-	margin-bottom: 15px;
-}
-
 .item-info {
 	display: block;
 }