Can I safely remove this sill?
I don't see why you couldn't go with the plan you are thinking of. It's probably concrete under it so it'll take a bit of work to get it off. You could consider using an angle grinder with a diamond blade and cut a groove across the front of the door to ensure any pieces that do break off don't break off under the door. probably will have to do this a few times until you get to the level you want.