XMLModuleStore should use HiddenDescriptor instead of RawDescriptor.

In https://github.com/edx/edx-platform/pull/25955 `HiddenDescriptor`
(which was a subclass of `RawDescriptor` with a custom `student_view()`)
was converted to an XBlock. It is used as the `default_class` by the
`CachingDescriptorSystem` classes. However `RawDescriptor` is still
being used by `XMLModuleStore`. This has been replaced by
`HiddenDescriptor` as well.

cms/djangoapps/contentstore/tests/test_courseware_index.py

@@ -101,7 +101,7 @@ class MixedWithOptionsTestCase(MixedSplitTestCase):
     DATABASE = 'test_mongo_%s' % uuid4().hex[:5]
     COLLECTION = 'modulestore'
     ASSET_COLLECTION = 'assetstore'
-    DEFAULT_CLASS = 'xmodule.raw_module.RawDescriptor'
+    DEFAULT_CLASS = 'xmodule.hidden_module.HiddenDescriptor'
     RENDER_TEMPLATE = lambda t_n, d, ctx=None, nsp='main': ''
     modulestore_options = {
         'default_class': DEFAULT_CLASS,