Remove redundant amplitudes if fewer than three subsystems

[redeboer]

If there are two subsystems, ampform-dpd still creates amplitudes for all three subsystems. See for instance the J/ψ → K⁰ Σ⁺ p̅ notebook. The model is still correct, because the amplitudes form the non-existent subsystems are set to 0, but it is still counterintuitive.