video: fbdev: atari: Remove unused atafb_setcolreg()

atafb_probe() overrides the atafb_ops.fb_setcolreg() method to match the
actual graphics hardware.  Besides, the shifts by 8 were bogus, as the
individual .fb_setcolreg() implementations already take care of that.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Helge Deller <deller@gmx.de>