History: Remove unimplemented function declaration.

Change-Id: I8b65fa141781adc4016563bf54c6f621959ecc79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518323
Commit-Queue: Scott Violet <sky@chromium.org>
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823815}

components/history/core/browser/url_database.h

@@ -246,13 +246,6 @@ class URLDatabase {
   // removed once data is collected from experiment.
   virtual bool GetVisitsForUrl2(URLID url_id, VisitVector* visits);
 
-  // Migration -----------------------------------------------------------------
-
-  // Do to a bug we were setting the favicon of about:blank. This forces
-  // about:blank to have no icon or title. Returns true on success, false if
-  // the favicon couldn't be updated.
-  bool MigrateFromVersion11ToVersion12();
-
  protected:
   friend class VisitDatabase;