Fix panic caused by despawning entity with ColliderOf

[Jondolf]

Objective

Fixes #676.

#671 changed the ColliderParent component to a ColliderOf relationship. Because Bevy does not normally allow relationships to point to their own entities, this required a partial manual implementation of the Relationship trait. However, I left the default on_replace implementation, which (as it turns out) panics if the target entity does not exist, which is possible when despawning an entity and the relationship points to itself.

Solution

Manually implement on_replace in a non-panicking way. Also replace some panicking uses of remove with try_remove.

Ideally this would be fixed by Bevy allowing self-relationships though.

[alec-deason]

Looks good to me. I can verify this fixes both my minimal case and the more complex case it was derived from.