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