Remove dedicated storage for NuanceRequestor

Summary:
Ref T10537. Currently, Nuance has a NuanceRequestor object, intended to represent the external user who created content (e.g., a GitHub account or a Twitter account or whatever).

This object is currently almost unused, and its design predates Doorkeeper. In D15541, I chose to use doorkeeper objects instead of NuanceRequestor objects to represent requestors.

I don't currently anticipate a need for such an object, given that we have Doorkeeper. If we do need it in the future for some reason, it would be fairly easy to restore it, create a requestor type which wraps a Doorkeeper object, and then migrate. Not super thrilling to do that, but not a huge mess.

NuanceItem still has a requestorPHID, but this is now a less formal object PHID instead of a more formal Requestor-object PHID, and holds a doorkeeper exeternal object PHID for GitHub events.

Test Plan:

• Grepped for nuancerequestor.
• Ran bin/storage upgrade -f.
• Grepped for requestor, remaining uses of this term seem reasonable/correct.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10537

Differential Revision: https://secure.phabricator.com/D15546