How to get user's photo url without transform?
According to the Craft 3 source code, User::getPhotoUrl() has been deprecated. Use getPhoto() to access the photo asset (if there is one), and call its getUrl() method to access the photo URL. (Also mentioned in the changelog.)

So assuming you've set user to something already in your template, the correct way to get the photo in Craft 3 is:

{{ user.photo.url() }}