Fix empty-section.rs test

The Auto Trait Implementation section is not wrapped in a
`synthetic-implementations` class. In fact, it is wrapped in a
`synthetic-implementations` id. However, we can generalize and completely remove
the `synthetic-implementations` requirement. We just have to verify that there's
no mention of "Auto Trait Implementations" anywhere.

src/test/rustdoc/empty-section.rs

@@ -15,6 +15,6 @@
 pub struct Foo;
 
 // @has foo/struct.Foo.html
-// @!has - '//*[@class="synthetic-implementations"]' 'Auto Trait Implementations'
+// @!has - 'Auto Trait Implementations'
 impl !Send for Foo {}
 impl !Sync for Foo {}