Prerender: Remove unused utility function declarations

These functions are declared in prerender_util.h, but not defined.

- ReportPrerenderExternalURL()
- ReportUnsupportedPrerenderScheme()

According to the Git history, the definitions were removed by this CL:
https://crrev.com/c/c2157903

Bug: 755921
Change-Id: Id070c760d5769be2f97b36213e4fd9360a01fd94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2465355
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Auto-Submit: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816309}

components/prerender/browser/prerender_util.h

@@ -20,12 +20,6 @@ class PrerenderManager;
 // Indicates whether the URL provided is a GWS origin.
 bool IsGoogleOriginURL(const GURL& origin_url);
 
-// Report a URL was canceled due to trying to handle an external URL.
-void ReportPrerenderExternalURL();
-
-// Report a URL was canceled due to unsupported prerender scheme.
-void ReportUnsupportedPrerenderScheme(const GURL& url);
-
 // Records the metrics for the nostate prefetch to an event with UKM source ID
 // |source_id|.
 void RecordNoStatePrefetchMetrics(