Remove comment about Firefox interpreting an empty string as a relative URL

The spec doesn't say an empty string shouldn't be prefetched, cf. https://bugzilla.mozilla.org/show_bug.cgi?id=1530716#c2

instantpage.js

@@ -124,7 +124,5 @@ function preload(url) {
 }
 
 function stopPreloading() {
-  /* The spec says an empty string should abort the prefetching
-  * but Firefox 64 interprets it as a relative URL to prefetch. */
   prefetcher.removeAttribute('href')
 }