R1:f50693de61d9

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…