The fixed graph for deleting all wall instances.

DeleteAllWalls-Complete.py

import clr
clr.AddReference('RevitAPI')
from Autodesk.Revit.DB import FilteredElementCollector, BuiltInCategory

clr.AddReference('RevitServices')
from RevitServices.Persistence import DocumentManager
from RevitServices.Transactions import TransactionManager

doc = DocumentManager.Instance.CurrentDBDocument
walls = FilteredElementCollector(doc).OfCategory(BuiltInCategory.OST_Walls).WhereElementIsNotElementType().ToElements()

TransactionManager.Instance.EnsureInTransaction(doc)
for wall in walls:
    doc.Delete(wall.Id)
TransactionManager.Instance.TransactionTaskDone()

OUT = "Success"