make TweenAnim removal in step_impl infallible

[adsick]

The motivation is that in our project we use this construct

.spawn(...)
.observe(|trigger: On<AnimCompletedEvent>, mut commands: Commands| {
    commands.entity(trigger.event().event_target()).try_despawn();
});

to despawn entities when animation completes, but current implementation in bevy_tweening expects that entity exists. I'm a little puzzled by the fact that our observer runs before your component cleanup, but this PR fixes it anyway.

[extrawurst]

Thanks for finding the cause of these panics, however your fix was not sufficient in my case. see #167

[djeedai]

This should be fixed by #167, closing this.