Remove frontend 'DEFAULT_CORPUS_ATTRS' mention

'DEFAULT_CORPUS_ATTRS' is not actually used in the frontend anymore, so this mention is useless / confusing now.

arkindex/project/default_corpus.py

 """
 Default corpus attributes
-
-If you edit default types, please update 'DEFAULT_CORPUS_ATTRS' config value in the frontend (at js/config.js)
 """
 
 DEFAULT_CORPUS_TYPES = [