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

iota.py/iota/commands/extended/is_promotable.py

Line 96 in a68254d

if response['promotable']:

Originally posted by @lzpap in #261