Remove unused variables

* Remove unused IAM varialbes and `has set IAM Lifecycle handler` variables

Author: tanaynigam

android/src/main/java/com/onesignal/flutter/OneSignalPlugin.java

@@ -59,11 +59,6 @@ public class OneSignalPlugin
   private boolean hasSetRequiresPrivacyConsent = false;
   private boolean waitingForUserPrivacyConsent = false;
 
-  private OSInAppMessage inAppMessage;
-  private boolean hasSetOnWillDisplayInAppMessageHandler = false;
-  private boolean hasSetOnDidDisplayInAppMessageHandler = false;
-  private boolean hasSetOnWillDismissInAppMessageHandler = false;
-  private boolean hasSetOnDidDismissInAppMessageHandler = false;
 
   private final HashMap<String, OSNotificationReceivedEvent> notificationReceivedEventCache = new HashMap<>();