Fix VirtualizedSectionList._setUpdateHighlightFor deleting from wrong target (memory leak)

## Summary

Fix a typo in _setUpdateHighlightFor where the else branch (called on cell unmount to clean up) was calling:

delete this._updateHighlightFor[cellKey];

This targeted this._updateHighlightFor ## Summary

Fix a typo in _setUpdateHighlightFor where the else branch (called on cell unmount to clean up) was calling:

delete this._updateHighlightFor[cellKey];

This targeted this._updateHighlightFor - the arrow function method itself - rather than this._updateHighlightMap, the backing store object. The FlowFixMe suppression comment was silencing the resulting Flow type error.

Fix:
delete this._updateHighlightMap[cellKey];

## Impact

- Memory leak: highlight callbacks accumulated in _updateHighlightMap and were never removed on unmount
- - Stale callbacks: could trigger highlight state updates on cells that no longer exist
- - The sibling method _setUpdatePropsFor always had the correct pattern
## Test Plan
- Existing separator highlight tests cover this code path
- - Typo fix restoring originally intended behavior
## Changelog

[General] [Fixed] - Fix VirtualizedSectionList._setUpdateHighlightFor deleting from wrong target (memory leak)

Differential Revision: D127652888

Pull Request resolved: #59455

Author: bl4987637-code

packages/virtualized-lists/Lists/VirtualizedSectionList.js

@@ -431,8 +431,7 @@ class VirtualizedSectionList<
     if (updateHighlightFn != null) {
       this._updateHighlightMap[cellKey] = updateHighlightFn;
     } else {
-      // $FlowFixMe[prop-missing]
-      delete this._updateHighlightFor[cellKey];
+      delete this._updateHighlightMap[cellKey];
     }
   };