[ads][CodeHealth] Remove stale kMaximumNotificationAdsPerHour pref migration (#36768)

The migration that cleared kMaximumNotificationAdsPerHour and opted the
user out of notification ads when the value was zero was added in
08/2023. The migration has been in place long enough that all affected
users have upgraded, so remove it.

components/brave_ads/browser/ads_service_impl.cc

@@ -220,14 +220,6 @@ void AdsServiceImpl::RegisterResourceComponents() {
 }
 
 void AdsServiceImpl::Migrate() {
-  // Added 08/2023.
-  const int64_t ads_per_hour =
-      prefs_->GetInt64(prefs::kMaximumNotificationAdsPerHour);
-  if (ads_per_hour == 0) {
-    prefs_->ClearPref(prefs::kMaximumNotificationAdsPerHour);
-    prefs_->SetBoolean(prefs::kOptedInToNotificationAds, false);
-  }
-
   // Added 10/2025.
   if (!local_state_->HasPrefPath(prefs::kFirstRunAt)) {
     base::Time first_run_at =