Remove constructor constraint from interface declaration

[fabre-thibaud]

Fix #164

[weaverryan]

+1 on this fix!

And removing this doesn't hurt anything - it's obviously not used anywhere (e.g. it's not like we're type-hinting an ApiInterface argument and then calling __construct on it. The only usage is when we're constructing the Api\* classes in Client, and these are all concrete classes that we're instantiating anyways, where we definitely know what the constructor looks like (e.g. new Api\Organization($this);).

👍

[Nek-]

Thank you @aztech-dev