Fix wrong variable in highlight callback cleanup

_setUpdateHighlightFor deletes from this._updateHighlightFor (a method)
instead of this._updateHighlightMap (the actual map). This means
highlight callbacks are never cleaned up when cells are removed,
causing a memory leak. The $FlowFixMe annotation was masking
the type error.

Author: Nedunchezhiyan-M

packages/virtualized-lists/Lists/VirtualizedSectionList.js

@@ -433,8 +433,7 @@ class VirtualizedSectionList<
     if (updateHighlightFn != null) {
       this._updateHighlightMap[cellKey] = updateHighlightFn;
     } else {
-      // $FlowFixMe[prop-missing]
-      delete this._updateHighlightFor[cellKey];
+      delete this._updateHighlightMap[cellKey];
     }
   };