There’s a way for not showing the close button (X) in the windowed mode?
And for positioning the window on the top (the upper edge) of the screen?
The position of the Game’s Window is entirely a client-side decision.
After going into Edit → Project Settings → Player Settings and taking a quick look through all of the Player Settings (“player” means the actual Unity Game Player, not the client who is using the game), I see that there is no option to disable the Closing button.
All options that involve the Player are handled through the Player Settings. Therefore, it is safe to assume that there is no way to disable the Closing button.