revert part of Tons commit r33884.

- rather then use unlink="None", just don't pass unlink as a keyword. This is more pythonic.
- added an RNA flag for properties which cant be unlinked by setting to None.

release/scripts/ui/properties_object.py

@@ -37,7 +37,7 @@ class OBJECT_PT_context_object(ObjectButtonsPanel, bpy.types.Panel):
         ob = context.object
 
         if space.use_pin_id:
-            layout.template_ID(space, "pin_id", unlink="None")
+            layout.template_ID(space, "pin_id")
         else:
             row = layout.row()
             row.label(text="", icon='OBJECT_DATA')