Fabric: clean up oldChildShadowView when handle Insert mount (#41155)

Summary:
We don't need to get oldChildShadowView when we handle the insert mount. So we can remove it.

## Changelog:

[IOS] [CHANGED] - Fabric: clean up oldChildShadowView when handling Insert mount

Pull Request resolved: #41155

Test Plan: None.

Reviewed By: sammy-SC

Differential Revision: D50554037

Pulled By: javache

fbshipit-source-id: 3250b6bbe119d800f05f39f790dc6949357d4f27

packages/react-native/React/Fabric/Mounting/RCTMountingManager.mm

@@ -73,7 +73,6 @@ static void RCTPerformMountInstructions(
       }
 
       case ShadowViewMutation::Insert: {
-        auto &oldChildShadowView = mutation.oldChildShadowView;
         auto &newChildShadowView = mutation.newChildShadowView;
         auto &parentShadowView = mutation.parentShadowView;
         auto &newChildViewDescriptor = [registry componentViewDescriptorWithTag:newChildShadowView.tag];
@@ -83,11 +82,11 @@ static void RCTPerformMountInstructions(
 
         RCTAssert(newChildShadowView.props, @"`newChildShadowView.props` must not be null.");
 
-        [newChildComponentView updateProps:newChildShadowView.props oldProps:oldChildShadowView.props];
+        [newChildComponentView updateProps:newChildShadowView.props oldProps:nullptr];
         [newChildComponentView updateEventEmitter:newChildShadowView.eventEmitter];
-        [newChildComponentView updateState:newChildShadowView.state oldState:oldChildShadowView.state];
+        [newChildComponentView updateState:newChildShadowView.state oldState:nullptr];
         [newChildComponentView updateLayoutMetrics:newChildShadowView.layoutMetrics
-                                  oldLayoutMetrics:oldChildShadowView.layoutMetrics];
+                                  oldLayoutMetrics:EmptyLayoutMetrics];
         [newChildComponentView finalizeUpdates:RNComponentViewUpdateMaskAll];
 
         [parentViewDescriptor.view mountChildComponentView:newChildComponentView index:mutation.index];