Delete items by their asset_key instead of _id.

The _id comes back as a dict and it's in the wrong order compared to the
dict stored in mongo.  This results in lookups for deletion failing when
we use the '_id' as the lookup key.

Luckily the delete function also takes the asset_key as a valid key to
delete by so we just use that instead.

cms/djangoapps/contentstore/tests/tests.py

@@ -335,7 +335,8 @@ class AuthTestCase(ContentStoreTestCase):
         is turned off
         """
         response = self.client.get(reverse('login'))
-        self.assertNotContains(response,
+        self.assertNotContains(
+            response,
             '<a href="/signup" class="action action-signin">Don&#39;t have a Studio Account? Sign up!</a>'
         )