I hope not. I say it to my dogs quite often. To a person I might say "Let go!" or "stop fretting about it". I believe Pt can emphasize the "it" should it wish to "Deixe-o", and so can English "Leave it" but it doesn't have to and "Leave" is not wrong. Confusion with sair would not arise in practice so Deixa could not mean "Go out" or "Take yourself off!". As JHeaven says "Saia, sai or saí, for that meaning.