strip cell.trusted as a transient value

We write trust to cell.metadata.trusted now,
but used to write it to cell.trusted.
Just make sure we never get either from disk.

IPython/nbformat/v3/rwbase.py

@@ -150,6 +150,9 b' def strip_transient(nb):'
     for ws in nb['worksheets']:
         for cell in ws['cells']:
             cell.get('metadata', {}).pop('trusted', None)
+            # strip cell.trusted even though it shouldn't be used,
+            # since it's where the transient value used to be stored.
+            cell.pop('trusted', None)
     return nb