Remove SDL_SetWindowGrab for SDL3

[cgutman]

We split SDL_SetWindowGrab() into SDL_SetWindowMouseGrab() and SDL_SetWindowKeyboardGrab() back in SDL 2.0.16 to provide a nicer interface than needing to mess around with SDL_HINT_GRAB_KEYBOARD to get keyboard grab (and having no way to grab the keyboard without also grabbing the mouse).

The corresponding window flag (SDL_WINDOW_INPUT_GRABBED) is already gone for SDL3, so I think we should also do away with the rest of the old grab interface (SDL_SetWindowGrab(), SDL_GetWindowGrab(), and SDL_HINT_GRAB_KEYBOARD) to force applications to be explicit about what type of grab they want.

[slouken]

Sure, go ahead and create a PR for that.