Fix styling of [source] links

Now that the `.viewcode-link` selector is being used, it's not
necessary to try to use `+` to find an adjacent element. The prior
revision dropped this, but did not remove the selectors for elements
neighboring the viewcode-link, resulting in the style not being
applied.

src/furo/assets/styles/content/_api.sass

@@ -56,9 +56,7 @@ dl.exception
       padding-right: 0.25rem
 
     // adjust the size of the [source] link on the right.
-    code.sig-name.descname,  // final part of named object.
-    span.sig-paren,          // final ')' on a method definition
-    > span.pre               // return annotation, on a function definition
+    a.reference.internal
       .viewcode-link
         width: 3.5rem
         font-size: var(--font-size--small)