video: fbdev: matroxfb: remove dead code and set but not used variable

Fix the following gcc warning:

drivers/video/fbdev/matrox/g450_pll.c:336:15: warning: variable
‘pixel_vco’ set but not used [-Wunused-but-set-variable]
  unsigned int pixel_vco;
               ^~~~~~~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200403021609.20968-1-yanaijie@huawei.com