video: fbdev: atari: Remove unused definitions and variables

Several definitions and variables are unused.
Some variables are set but further unused.

Remove them.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>