Remove SDL_VIDEODRIVER from startxfce4

SDL_VIDEODRIVER makes SDL Applications crash if they don't support the native backend. Why is it set anyway per sdl2 documantation if it is not set SDL2 would figure out what to start. https://github.com/libsdl-org/SDL/blob/3c08172d0626b92966ce54aea03f6fa7e5dd9c84/include/SDL_hints.h#L3008

As for SDL3 it is entirely diff variable: https://github.com/libsdl-org/SDL/blob/8d604353a53853fa56d1bdce0363535605ca868f/include/SDL3/SDL_hints.h#L3313

but SDL3 still figures out what backend to start.