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/struct-arg-pattern.rs

@@ -4,7 +4,7 @@ struct BodyId {
     hir_id: usize,
 }
 
-// @has 'foo/fn.body_owner.html' '//*[@class="rust fn"]' 'pub fn body_owner(_: BodyId)'
+// @has 'foo/fn.body_owner.html' '//div[@class="item-decl"]/pre[@class="rust"]' 'pub fn body_owner(_: BodyId)'
 pub fn body_owner(BodyId { hir_id }: BodyId) {
     // ...
 }