Cleanup unnecessary code in brave_stats_updater_browsertest.cc

Since we now acknowledge for blocking operations when running tests
due to BraveStatsUpdaterParams::GetFirstRunTime(), we no longer need
to explicitly call BraveStatsUpdaterParams::SetFirstRunForTest(true)
from brave_stats_updater_browsertest.cc, so let's remove that too.

browser/brave_stats/brave_stats_updater_browsertest.cc

@@ -58,9 +58,6 @@ class BraveStatsUpdaterBrowserTest : public InProcessBrowserTest {
     brave::RegisterPrefsForBraveReferralsService(
         testing_local_state_.registry());
     SetBaseUpdateURLForTest();
-    // Simulate sentinel file creation as if chrome_browser_main.h was called,
-    // which reads in the sentinel value and caches it.
-    brave_stats::BraveStatsUpdaterParams::SetFirstRunForTest(true);
   }
 
   void SetUpCommandLine(base::CommandLine* command_line) override {

browser/brave_stats/brave_stats_updater_params.h

@@ -12,7 +12,6 @@
 #include "base/time/time.h"
 #include "brave/components/brave_stats/browser/brave_stats_updater_util.h"
 
-class BraveStatsUpdaterBrowserTest;
 class BraveStatsUpdaterTest;
 class PrefService;
 
@@ -44,7 +43,6 @@ class BraveStatsUpdaterParams {
   void SavePrefs();
 
  private:
-  friend class ::BraveStatsUpdaterBrowserTest;
   friend class ::BraveStatsUpdaterTest;
   PrefService* stats_pref_service_;
   PrefService* profile_pref_service_;