Is chin implant removed in all cases before orthognathic surgery?

Either works. But often the implant is removed and a genioplasty performed to ensure proper profile contours. The chin has a muscle 'under' it which can 'stretch' forward with a genioplasty. That muscle is the attachment of the tongue which you want to move forward if you have sleep apnea. The actual situation is best discussed with the surgeon since there are other factors which would determine the final plan