Stop passing cobalt_config_dir to process_starter.start_test_app()

Since this parameter was removed.

CB-102

Change-Id: Ibdb79aab35ddbaac3e1c69c0702b2936401e87ac
diff --git a/cobaltb.py b/cobaltb.py
index d8368ea..9cc9fcb 100755
--- a/cobaltb.py
+++ b/cobaltb.py
@@ -432,7 +432,6 @@
       use_tls=use_tls,
       root_certs_pem_file=args.shuffler_root_certs,
       shuffler_pk_pem_file=shuffler_public_key_pem,
-      cobalt_config_dir=args.cobalt_config_dir,
       project_id=args.project_id,
       # Because it makes the demo more interesting
       # we use verbose_count at least 3.