viewer: remove useless gtk2/3 switch.
viewer: remove useless gtk2/3 switch.

AnilRamachandran/aravis,AravisProject/aravis,AravisProject/aravis,AnilRamachandran/aravis,AnilRamachandran/aravis,AravisProject/aravis,AnilRamachandran/aravis,AravisProject/aravis,AnilRamachandran/aravis,AravisProject/aravis