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>'
         )
 

common/lib/xmodule/xmodule/contentstore/utils.py

@@ -16,13 +16,13 @@ def empty_asset_trashcan(course_locs):
         thumbs = store.get_all_content_thumbnails_for_course(course_loc)
         for thumb in thumbs:
             print("Deleting {0}...".format(thumb))
-            store.delete(thumb['_id'])
+            store.delete(thumb['asset_key'])
 
         # then delete all of the assets
         assets, __ = store.get_all_content_for_course(course_loc)
         for asset in assets:
             print("Deleting {0}...".format(asset))
-            store.delete(asset['_id'])
+            store.delete(asset['asset_key'])
 
 
 def restore_asset_from_trashcan(location):