Removed logic which was backfilling a default value for P3A

Fixes https://github.com/brave/brave-browser/issues/19324

android/java/org/chromium/chrome/browser/app/BraveActivity.java

@@ -477,12 +477,6 @@ public abstract class BraveActivity<C extends ChromeActivityComponent> extends C
         BraveSyncInformers.show();
         BraveAndroidSyncDisabledInformer.showInformers();
 
-        if (!PackageUtils.isFirstInstall(this)
-                && !OnboardingPrefManager.getInstance().isP3AEnabledForExistingUsers()) {
-            BravePrefServiceBridge.getInstance().setP3AEnabled(true);
-            OnboardingPrefManager.getInstance().setP3AEnabledForExistingUsers(true);
-        }
-
         if (!OnboardingPrefManager.getInstance().isOneTimeNotificationStarted()
                 && PackageUtils.isFirstInstall(this)) {
             RetentionNotificationUtil.scheduleNotification(this, RetentionNotificationUtil.HOUR_3);

android/java/org/chromium/chrome/browser/onboarding/OnboardingPrefManager.java

@@ -34,8 +34,6 @@ import java.util.Map;
 public class OnboardingPrefManager {
     private static final String PREF_ONBOARDING = "onboarding";
     private static final String PREF_P3A_ONBOARDING = "p3a_onboarding";
-    private static final String PREF_P3A_ENABLED_FOR_EXISTING_USERS =
-            "p3a_enabled_for_existing_users";
     private static final String PREF_CROSS_PROMO_MODAL = "cross_promo_modal";
     private static final String PREF_ONBOARDING_V2 = "onboarding_v2";
     private static final String PREF_NEXT_ONBOARDING_DATE = "next_onboarding_date";
@@ -109,16 +107,6 @@ public class OnboardingPrefManager {
         sharedPreferencesEditor.apply();
     }
 
-    public boolean isP3AEnabledForExistingUsers() {
-        return mSharedPreferences.getBoolean(PREF_P3A_ENABLED_FOR_EXISTING_USERS, false);
-    }
-
-    public void setP3AEnabledForExistingUsers(boolean isShown) {
-        SharedPreferences.Editor sharedPreferencesEditor = mSharedPreferences.edit();
-        sharedPreferencesEditor.putBoolean(PREF_P3A_ENABLED_FOR_EXISTING_USERS, isShown);
-        sharedPreferencesEditor.apply();
-    }
-
     /**
      * Returns the user preference for whether the onboarding is shown.
      */