Use indices instead of WorkTimeDatesInterface

Checking start/end dates are done very, very often in date calculations (often looping through every day in a date range). So I'd prefer more opaque code but less overhead.

src/ifcopenshell-python/ifcopenshell/util/data.py

@@ -93,37 +93,3 @@ class Clipping:
 
         second_operand = ifc_file.createIfcHalfSpaceSolid(plane, False)
         return ifc_file.createIfcBooleanClippingResult("DIFFERENCE", first_operand, second_operand)
-
-
-class WorkTimeDatesInterface:
-    def __init__(self, work_time: ifcopenshell.entity_instance):
-        """Utility class for IfcWorkTime dates allowing refer
-        to it's dates as `.Start`/`.Finish` or `.StartDate`/`.FinishDate`
-        regardless of the schema version
-        """
-        if hasattr(work_time, "StartDate"):  # since IFC4X3
-            self.start_attr, self.finish_attr = "StartDate", "FinishDate"
-        else:
-            self.start_attr, self.finish_attr = "Start", "Finish"
-
-        self.work_time = work_time
-
-    @property
-    def Start(self):
-        return getattr(self.work_time, self.start_attr)
-
-    @Start.setter
-    def Start(self, value):
-        return setattr(self.work_time, self.start_attr, value)
-
-    StartDate = Start
-
-    @property
-    def Finish(self):
-        return getattr(self.work_time, self.finish_attr)
-
-    @Finish.setter
-    def Finish(self, value):
-        return setattr(self.work_time, self.finish_attr, value)
-
-    FinishDate = Finish