fbdev: tridentfb: Remove the unused function shadowmode_off()

The function shadowmode_off() is defined in the tridentfb.c file, but not
called elsewhere, so delete this unused function.

drivers/video/fbdev/tridentfb.c:1131:20: warning: unused function 'shadowmode_off'.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2154


Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Helge Deller <deller@gmx.de>