The word "Unneeded" is a lot rarer than "Unnecessary", according to Google ngrams. The word "Unneeded" has also been used less in older times than current times, which would be consistent with it being a non-standard word.
oxforddictionaries.com and Wiktionary have definitions for "Unneeded", and don't describe it as non-standard. But I'm not certain that it is standard English.
Are people likely to regard "Unneeded" as non-standard, or as a newspeak style neologism based on combining "un" with "needed" by someone who doesn't know how to spell "Unnecessary"?