Disable automatic creation of ClosestToBeam state for downstream tracks in TrackMasterFitter

The ClosestToBeam state for downstream tracks doesn't carry the same semantic meaning as it does for long tracks.
For that reason this MR for now disables this state creation as it's unclear what the right thing to do is here.
(This also serves as a test to make sure nothing relies on this state right now ;) )

In addition, it makes using parametrized scatters very complicated because the position of the ClosesToBeam state could end up anywhere along the z-axis

cc @ldufour