First attempt at ripping out agnostic code into ifcopenshell.api namespace. See #1399.

src/ifcopenshell-python/ifcopenshell/api/classification/remove_reference.py

@@ -0,0 +1,28 @@
+import ifcopenshell.util.schema
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {"reference": None, "product": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        total_related_objects = 0
+        for association in self.file.by_type("IfcRelAssociatesClassification"):
+            if association.RelatingClassification == self.settings["reference"] and association.RelatedObjects:
+                total_related_objects += len(association.RelatedObjects)
+                related_objects = list(association.RelatedObjects)
+                try:
+                    related_objects.remove(self.settings["product"])
+                except:
+                    continue
+                if len(related_objects):
+                    association.RelatedObjects = related_objects
+                else:
+                    self.file.remove(association)
+
+        # TODO: we only handle lightweight classifications here
+        if total_related_objects == 1:
+            self.file.remove(self.settings["reference"])