Remove sleep in feature_config_args

Summary:
I can't remember why it was here in the first place but it's clearly not
needed anymore.

Test Plan:

./test/functional/test_runner.py feature_config_args

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3083