video: fbdev: intelfb: remove redundant variables

Variables err_max, err_target and f_best are being assigned values but
these are never read, hence they are redundant variables and can be
removed. Cleans up clang warnings:

drivers/video/fbdev/intelfb/intelfbhw.c:946:2: warning: Value stored to
'err_max' is never read
drivers/video/fbdev/intelfb/intelfbhw.c:947:2: warning: Value stored to
'err_target' is never read
drivers/video/fbdev/intelfb/intelfbhw.c:995:6: warning: Value stored to
'f_best' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Cc: Maik Broemme <mbroemme@libmpq.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>