Clean up usage of isImmersive

So that we don't set both isImmersive and allowGestureIgnoringBarVisibility

Follow up of ag/q/topic:%22remove_isimmersive%22

Bug: 313033827
Change-Id: I56026906bfbdf4584e7c1f189d60520d61640991
Test: N/A

packages/SystemUI/src/com/android/systemui/navigationbar/TaskbarDelegate.java

@@ -495,10 +495,6 @@ public class TaskbarDelegate implements CommandQueue.Callbacks,
         return mBehavior != BEHAVIOR_SHOW_TRANSIENT_BARS_BY_SWIPE;
     }
 
-    private boolean isImmersiveMode() {
-        return mBehavior == BEHAVIOR_SHOW_TRANSIENT_BARS_BY_SWIPE;
-    }
-
     public void onConfigurationChanged(Configuration configuration) {
         mEdgeBackGestureHandler.onConfigurationChanged(configuration);
     }