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 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`.

tests/rustdoc/hide-complex-unevaluated-const-arguments.rs

@@ -63,7 +63,7 @@ impl<const S: Struct, St: Stage + ?Sized> Helper<S> for St {
 // this test as long as one can ensure that private fields are not leaked!
 //
 // @has hide_complex_unevaluated_const_arguments/trait.Sub.html \
-//      '//*[@class="rust trait"]' \
+//      '//div[@class="item-decl"]/pre[@class="rust"]' \
 //      'pub trait Sub: Sup<{ _ }, { _ }> { }'
 pub trait Sub: Sup<{ 90 * 20 * 4 }, { Struct { private: () } }> {}