Remove forward-declaration of non-existent class

GoogleURLTrackerNavigationHelper class was completely removed as part
of https://codereview.chromium.org/808253006/ but a forward-declaration
was left in the code, remove it.

BUG=None
R=isherman@chromium.org

Review URL: https://codereview.chromium.org/929483005

Cr-Commit-Position: refs/heads/master@{#317466}

components/google/core/browser/google_url_tracker.h

@@ -16,7 +16,6 @@
 #include "net/url_request/url_fetcher_delegate.h"
 #include "url/gurl.h"
 
-class GoogleURLTrackerNavigationHelper;
 class PrefService;
 
 namespace infobars {