BusinessProcess: Fix getPreviousPhaseDict() for BL with >1 children/parents BLs.

Called by TransformationSimulationRule and did not work with the
following use case (where child_BL1):

  BL(trade_phase=TP1, successor=trade_state/TS1):
    * child_BL1(predecessor=trade_state/TS1,
                trade_phase=TP1/InventoryAccounting)
    * child_BL3(predecessor=trade_state/TS1,
                trade_phase=Transforming)

product/ERP5/Document/BusinessProcess.py

@@ -875,11 +875,15 @@ class BusinessProcess(Path, XMLObject):
         # i.e. edit next phases to replace current phase by previous ones
         for next in next_set:
           phase_set = result[next]
-          phase_set.remove(phase)
+          # Not remove() as it may have already been removed earlier
+          # if >1 elements of next_set have the same parent
+          phase_set.discard(phase)
           phase_set |= previous_set
         # and previous phases to replace current by next ones
         for previous in previous_set:
           phase_set = next_dict[previous]
-          phase_set.remove(phase)
+          # Not remove() as it may have already been removed earlier
+          # if >1 elements of previous_set have the same child
+          phase_set.discard(phase)
           phase_set |= next_set
     return result