Make ``is_promotable`` response consistent with ``check_consistency``

[lzpap]

We can't control the response from the node in check_consistency, so to make it consistent, I suggest we implement the same behavior in is_promotable, that is to do

if response['promotable']:
    del response['info']

instead of the current response['info']=None in https://github.com/iotaledger/iota.py/blob/a68254d040dcf9a581a851f5953e056ff538897f/iota/commands/extended/is_promotable.py#L96

Originally posted by @lzpap in #261