I suppose you would have to dispose all the things using it and null all references to it.
SkeletonDataAsset itself doesn't hold references to the Texture2D. Though in the SkeletonData object, and its Skins, each individual Attachment does hold a reference to the Material needed to render it.
The AtlasAsset also has the Material references to allow it to load.
I would think you would release it like you would any other material and texture.