Remove need for binary_serialize_macaroons

[nottrobin]

The binary_serialize_macaroons function that we appear to need for authenticating with the advantage contracts service isn't needed in our other macaroon login flows.

It would be great to standardise our macaroon login tooling. Can we see if it's possible to avoid this in the contracts API? Maybe we could work with the contracts API team to figure it out.

[petesfrench]

@samhotep Would it be possible for you to look into this? It looks to me like it is no longer an issue