don't write cell.trusted to disk

Notary.check_cells should pop the keys, not just read them.

IPython/nbformat/sign.py

@@ -203,12 +203,13 class NotebookNotary(LoggingConfigurable):
         """
         if not nb['worksheets']:
             return True
+        trusted = True
         for cell in nb['worksheets'][0]['cells']:
             if cell['cell_type'] != 'code':
                 continue
-            if not cell.get('trusted', False):
-                return False
-        return True
+            if not cell.pop('trusted', False):
+                trusted = False
+        return trusted
 
 
 trust_flags = {

IPython/nbformat/tests/test_sign.py

@@ -103,6 +103,10 class TestNotary(TestsBase):
         nb = self.nb
         self.notary.mark_cells(nb, True)
         self.assertTrue(self.notary.check_cells(nb))
+        for cell in nb.worksheets[0].cells:
+            self.assertNotIn('trusted', cell)
         self.notary.mark_cells(nb, False)
         self.assertFalse(self.notary.check_cells(nb))
+        for cell in nb.worksheets[0].cells:
+            self.assertNotIn('trusted', cell)