Asset players in LinkedAnimInstances are ticked twice when a PostProcessAnimInstance is present

Reported here: https://udn.unrealengine.com/questions/569091/animation-playrate-in-linkedanimgraph.html

I tested with a downloaded copy of 4.24. It looks like this would also happen in 4.25 though.

UAnimInstance::ParallelUpdateAnimation() does this:

// Tick asset players for this and any linked instances we have
for(UAnimInstance* LinkedInstance : GetSkelMeshComponent()->GetLinkedAnimInstances())}}
{
    LinkedInstance->GetProxyOnAnyThread<FAnimInstanceProxy>().TickAssetPlayerInstances();
}

That gets called for both the regular anim instance and the post process anim instance.

A minimal repro sample project was provided on this UDN: https://udn.unrealengine.com/questions/569091/animation-playrate-in-linkedanimgraph.html

Their setup:

1. Main animbp has a Linked Anim Graph node
2. The linked anim graph has a sequence player
3. The post process anim graph just passes through the input pose