Amaranth: Fix addon un-register function

The issue was introduced in cf773d42f0 and vis caused
by a lack of class unregister.

Fixes regression tests, as well as usecase when a user
toggles the addon off and on again.

amaranth/misc/toggle_wire.py

@@ -133,3 +133,7 @@ def register():
 
 def unregister():
     clear_properties()
+
+    from bpy.utils import unregister_class
+    for cls in classes:
+        unregister_class(cls)