Fix duplication of Pilz motion sequence trajectory points... the right way

[sea-bass]

In #2943, I submitted a band-aid fix for the Pilz motion sequence action server to de-duplicate trajectory time points due to requirements on the ros2_controllers Joint Trajectory Controller.

I did this by looking at the final packaged up trajectory and deduplicating points there, whereas it would be better to resolve the issue as the blended trajectory segments are being built up.