Professional term for "useless: should be omitted"

Consider extraneous, "Not belonging to, or dependent upon, a thing; without or beyond a thing; foreign", and synonyms like superfluous ("in excess of what is required or sufficient") or pleonastic ("Using an excessive number of words"). Other synonyms of superfluous include excessive, extra, supernumerary, surplus, unnecessary, extravagant, some of which apply.

My preference in this instance would be to use either prejudicial or detrimental as they convey the sense of 'harmful' as requested.

Sometimes English just doesn't contain a word with the precise meaning you desire.

That said, I offer irrelevant and unnecessary as options. I would contend that both are semantically equivalent to useless (which--as Jeff Atwood points out--is a still a perfectly good word for your situation), but they tend more to the negative in my mind.

I use redundant when someone has written something repetitive and unnecessary.

You could say, "Your last sentence detracts from an otherwise good paragraph. I think your point would be better made by deleting that sentence."