Prevent duplicate notifications for removed anchors in WebXRAnchorSystem

[RaananW]

Implement a mechanism to track removed anchors, preventing duplicate notifications when anchors are removed from the session. This change introduces an internal flag to indicate if an anchor has already been removed.

Fixes the issue presented here - https://forum.babylonjs.com/t/anchorsystem-anchor-remove-causes-crash-on-android/51672?u=raananw

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