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

src/ifcopenshell-python/ifcopenshell/api/layer/unassign_layer.py

@@ -0,0 +1,17 @@
+import ifcopenshell
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {
+            "item": None,
+            "layer": None,
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        assigned_items = set(self.settings["layer"].AssignedItems) or set()
+        assigned_items.remove(self.settings["item"])
+        self.settings["layer"].AssignedItems = list(assigned_items)