rustdoc: remove bad CSS font-weight on .impl, .method, etc

This line was added in c494a06064, because at
the time, the headers had these classes on them. Now, the headers are
children of the `<section>` with the class on it.

This commit also adds a test case, to make sure the srclink font weight does
not regress again.

src/librustdoc/html/static/css/rustdoc.css

@@ -187,7 +187,6 @@ h4.code-header {
 .impl-items .associatedtype,
 .methods .associatedtype {
 	flex-basis: 100%;
-	font-weight: 600;
 	position: relative;
 }