Maya glTF exporter writes out duplicate texture objects

[bghgary]

Probably also for Max.

For example, https://github.com/bghgary/glTF-Sample-Models/blob/flighthelmet-fix/2.0/FlightHelmet/glTF/FlightHelmet.gltf#L881 is a duplicate of https://github.com/bghgary/glTF-Sample-Models/blob/flighthelmet-fix/2.0/FlightHelmet/glTF/FlightHelmet.gltf#L866.

Source Maya project: https://github.com/bghgary/glTF-Sample-Models/tree/flighthelmet-fix/sourceModels/FlightHelmet