IntersectionObserver: Clean up shadow nodes after unobserve

Summary:
After unobserve is called, the shadow node actually isn't deleted from the `targetToShadowNode` WeakMap. So, if the element is kept around, the shadow node will leak.

We suspect this doesn't happen in practice. But, still it's good to just do this cleanup, just in case.

Changelog: [General][Fixed] Fix potential leak inside IntersectionObserver

Reviewed By: lunaleaps, yungsters

Differential Revision: D74361644

fbshipit-source-id: 1a5ccce1486aa200fc7318c689bfa887f713c284

Author: RSNara

packages/react-native/src/private/webapis/intersectionobserver/internals/IntersectionObserverManager.js

@@ -232,6 +232,7 @@ export function unobserve(
       );
       return;
     }
+    targetToShadowNodeMap.delete(target);
 
     NativeIntersectionObserver.unobserve(
       intersectionObserverId,
@@ -245,6 +246,7 @@ export function unobserve(
       );
       return;
     }
+    targetToTokenMap.delete(target);
 
     modernNativeIntersectionObserver.unobserve(
       intersectionObserverId,