Bug Remove unused code from nsPrefetchNode::AsyncOnChannelRedirect, r=bzbarsky

Looks like this was overlooked in bug 442803

uriloader/prefetch/nsPrefetchService.cpp

@@ -374,9 +374,6 @@ nsPrefetchNode::AsyncOnChannelRedirect(nsIChannel *aOldChannel,
     if (NS_FAILED(rv))
         return rv;
 
-    nsCOMPtr<nsICachingChannel> oldCachingChannel =
-        do_QueryInterface(aOldChannel);
-
     PRBool match;
     rv = newURI->SchemeIs("http", &match); 
     if (NS_FAILED(rv) || !match) {