video: Remove set but not used variable

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/video/fbdev/sis/300vtbl.h:1064:28: warning:
‘SiS300_CHTVVCLKSONTSC’ defined but not used [-Wunused-const-variable=]
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Li Heng <liheng40@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1600957106-13741-1-git-send-email-liheng40@huawei.com

drivers/video/fbdev/sis/300vtbl.h

@@ -1061,8 +1061,6 @@ static const unsigned char SiS300_CHTVVCLKUNTSC[]  = { 0x29,0x29,0x29,0x29,0x2a,
 
 static const unsigned char SiS300_CHTVVCLKONTSC[]  = { 0x2c,0x2c,0x2c,0x2c,0x2d,0x2b };
 
-static const unsigned char SiS300_CHTVVCLKSONTSC[] = { 0x2c,0x2c,0x2c,0x2c,0x2d,0x2b };
-
 static const unsigned char SiS300_CHTVVCLKUPAL[]   = { 0x2f,0x2f,0x2f,0x2f,0x2f,0x31 };
 
 static const unsigned char SiS300_CHTVVCLKOPAL[]   = { 0x2f,0x2f,0x2f,0x2f,0x30,0x32 };