Update comments to indicate "visual_style" parameter is unused

[AaronAtDuo]

The comments for "visual_style" in the Admin API client indicates the user should see the API docs for information. In fact, this parameter is unused and deprecated - the comments should be updated to reflect this.

duo_client_python/duo_client/admin.py

Line 142 in 5ed6201

'visual_style': <str:visual style>}

[AaronAtDuo]

Solved by #283