If it's for non-Hyper Trained trade value, I'd say toss. 31 speed is necessary so you don't lose speed ties 100% of the time against other Solgaleo, and other collectors of non-Hyper trained legendaries will be very critical about the stat that's boosted by the nature being 31.
That doesn't really mean anything in this situation, because you could always HT something like an Adamant 31/2/31/31/13/8 Solgaleo, and it'd be just as good.
Edit: Sorry if I wasn't clear on what I meant by that, I'll clarify. I would assume that a 31/30/31/17/31/30 Adamant Solgaleo would take a lot of time and effort to get. So settling on that just because you can Hyper train it would effectively make all that time they spent wasted, since in the first place, they could just settle on any random IVs, as long as it had an adamant nature.