The dress code for a place of employment is wholly unrelated to the dress code for an interview. If you were going for a job as a plasterer, would you attend the interview in overalls?
Wear a suit. If you’re really against this, ditch the jacket, but I’d advise against it. If they query your attire after you’d asked them about it then reply along the lines of “I know you said it didn’t matter but I wanted to make an effort.”
Wear a tie. Nothing screams “I don’t give a crap about you or your poxy job” louder than an open-necked shirt in an interview. The one and only exception to wearing a tie is if you’re wearing boobs instead.