Level Sequences- Overriding binding of a spawnable mid-playback doesn't despawn spawnable

Reported by user in UDN.

Overriding the binding of a spawnable actor track while the sequence is playing will result in 2 visual copies of the actor, the original spawnable, which doesn't get destroyed until the Sequence finishes, and the runtime actor. While the original spawnable is no longer bound to the tracks, it does persist.

1. Create a sequence with a spawnable actor
2. Add blueprint code to trigger the sequence, wait some time, and then bind the player to the spawnable actor track
3. Play the game and recognize 2 versions of the spawnable