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

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

@@ -0,0 +1,24 @@
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {"classification": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        references = self.get_references(self.settings["classification"])
+        for reference in references:
+            self.file.remove(reference)
+        self.file.remove(self.settings["classification"])
+        for rel in self.file.by_type("IfcRelAssociatesClassification"):
+            if not rel.RelatingClassification:
+                self.file.remove(rel)
+
+    def get_references(self, classification):
+        results = []
+        if not classification.HasReferences:
+            return results
+        for reference in classification.HasReferences:
+            results.append(reference)
+            results.extend(self.get_references(reference))
+        return results