rustdoc: remove redundant item kind class from `.item-decl > pre`
This class originated in the very first commit of `rustdoc_ng`, and was used to add a color border around the item decl based on its kind.
4fd061c426/src/rustdoc_ng/html/static/main.css (L102-L106)
The item decl no longer has a border, and there aren't any kind-specific styles in modern rustdoc's rendering of this UI item.
Most of this PR is updating test cases so that they use `item-decl` to find the `<pre>` tag instead of relying on the fact that the class name had `rust {kind}` in it while other `<pre>` tags only had class `rust`.
b90f62988d