ATConverter: Get primary vertex from CbmEvent object

In the analysis tree converter, the primary vertex used in the reconstructed event header was taken from the CbmEvent object directly. Within the STS tracks converter, it was called from the PrimaryVertex object (should be removed, see: https://redmine.cbm.gsi.de/issues/3423) directly from cbmsim tree. In event-based mode, the same information was stored in both cases, however, in time-based mode, the PrimaryVertex object contrained only on entry/TS of (0,0,0) for the vertex. This resulted in wrong calculation DCA calculations, see discussion in https://redmine.cbm.gsi.de/issues/3398.

Hereby, the primary vertex is taken from CbmEvent in all cases, such that the PrimaryVertex object is no longer used in AT Converter.

Redmine #3427