One thing the trim does typically is hide the edge of the drywall which usually, one would not want to look at because it's rough and ugly.
It's hard to imagine trim that doesn't extend (at least the thickness of the metal itself) below the surface of the ceiling.
If you want a trimless look, what you will probably need to do is apply drywall compound up inside the can itself a bit, and cover the edge between the can and the drywall. Then carefully hand-sand that edge into the shape you want. Don't forget to protect the light socket by deenergizing it and stuffing some non-conductive plastic into it during this work. When done, re-paint the ceiling, and paint up into the can as well. Either use paint that can handle the heat of the bulb or commit to using LED bulbs only.