fix: remove incorrect key type when filtering inaccessible blocks

Returning the list with a 'completion' string was added in 9bc0f85. However,
the `get_accessible_sequences` method expects a list of dicts with an 'id' key.

lms/djangoapps/course_home_api/outline/views.py

@@ -501,7 +501,7 @@ def filter_inaccessible_blocks(self, course_blocks, course_key):
             for section_data in course_sections:
                 section_data['children'] = self.get_accessible_sequences(
                     user_course_outline,
-                    section_data.get('children', ['completion'])
+                    section_data.get('children', [])
                 )
                 accessible_sequence_ids = {str(usage_key) for usage_key in user_course_outline.accessible_sequences}
                 for sequence_data in section_data['children']: