Unify doc-strings (remove type/rtype)

As this information is already provided in type hints and we don't need to maintain it in two places.

src/ifcopenshell-python/ifcopenshell/api/unit/add_derived_unit.py

@@ -43,7 +43,6 @@ def add_derived_unit(
 
     :param unit_type: A type of unit chosen from the list above. For
         example, choosing THERMALCONDUCTANCEUNIT will give you a Thermal conductance.
-    :type unit_type: str
     :param userdefinedtype: The user defined type in case of choosing USERDEFINED, or None for no
         user defined type.
     :param attributes: a dictionary of attribute names and values.