changed to match standard

[mattlockyer]

Standard specifies no return value for this callback method.

https://nomicon.io/Standards/NonFungibleToken/ApprovalManagement.html#approved-account-contract-interface

[austinabell]

hmm, strange, seems that nft_approve is forwarding any result from this call as its return. I don't think necessarily that this function signature violates the standard though, since adding a return doesn't break any call unless someone for some reason asserted that the return bytes were empty?

In any case, this is a bad signature because it only allows String return anyway. I won't weigh in on the standards part of this, but changing this from its current signature is something I've wanted to do before, but I will wait until it is agreed on the direction to go with this.

[mattlockyer]

Just found it confusing.

[agostbiro]

Thanks for opening this PR and raising the issue @mattlockyer ! We need to discuss more how to approach it, so I opened a new issue for it and will close this PR in favor of that. Hope you don't mind and it would be great if you could participate in the discussion in #1053.