用户在Map上固定了多个大头针。我只想显示最后一个大头针，你能帮我吗？
@objc函数选择Pin（Map手势：UI长按手势识别器）{

if mapGesture.state == .began {
        
        let touchPoint = mapGesture.location(in: self.mapview)
        let touchCoordinates = self.mapview.convert(touchPoint, toCoordinateFrom: self.mapview)
        choosenLatitude = touchCoordinates.latitude
        choosenLongtitude = touchCoordinates.longitude
        
        let annotation = MKPointAnnotation()
        annotation.coordinate = touchCoordinates
        annotation.title = forWhatText.text
        if forWhatText.text == "" {
            makeAlert(titleInput: "Error", messageInput: "Please fill in all the fields above!")
        } else if phoneName.text == "" {
            makeAlert(titleInput: "Error", messageInput: "Please fill in all the fields above!")
        } else if phoneNumber.text == "" {
            makeAlert(titleInput: "Error", messageInput: "Please fill in all the fields above!")
        } else if messageText.text == "" {
            makeAlert(titleInput: "Error", messageInput: "Please fill in all the fields above!")
        } else {
            mapview.addAnnotation(annotation)
            //saveButton.isEnabled = true
        }
    }
}