Technically, all fish have scales. But on many of them: trout, salmon, many salt-water fish, they are so small that we don't notice them. So they remain on in those cases.
But most of the time they are removed.
Descaling, btw, is a job best done outside, because scales can fly every which way. If for some reason I haven't scaled them where I caught them, I like doing this out in the garden, because they add nitrogen to the soil.