rustdoc: remove redundant item kind class from `.item-decl > pre`

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.

https://github.com/rust-lang/rust/blob/4fd061c426902b0904c65e64a3780b21f9ab3afb/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 commit 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`.