It might be argued that unnecessary should be translated 'utHa', but given the options I see already accepted, and the fact that I can't think of a context in which the two are not interchangeable, I think that would be an unnecessary distinction. Added.
Thanks for the report.
The dictionary definition of unnecessary is not necessary, so thinking of 'utHa' as unnecessary and 'utbe' as not necessary is, perhaps, too-rigid translation. Un- doesn't always equal -Ha'.
'utHa' may mean something more like moving away from being necessary. Not only is it not necessary, being necessary is antithetical. I can't imagine a context in which that would be used, but it's possible.