HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

So, it won't be possible to revert it completely. Because a window-based town view and a full-screen town view are different things, and it would look, to put it mildly, not very good with a window. In any case, it's not something that anyone really needs (especially me). And you should get used to it.