Remove FileContentsAction for --langs

Summary:
# Facebook:

Revert changes made in D24059296 (facebookresearch@0557ed8) since it breaks normal --langs usage. To specify languages in a file, use --lang-dict instead

Also update integration test params so it can catch this

Reviewed By: tangyuq

Differential Revision: D24224622

fbshipit-source-id: 292eeb86e02528128ced09f8165045be9c847c19

fairseq/data/multilingual/multilingual_data_manager.py

@@ -87,7 +87,6 @@ def add_args(parser):
             type=csv_str_list,
             help="a list of languages comma sperated languages which can appear in lang-pairs; "
             "note that the ordering determines language token IDs",
-            action=FileContentsAction,
         )
         parser.add_argument(
             "--lang-dict",