drm/panel: db7430: remove unused variables

These variables are never referenced in the code.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230719105829.148011-1-luca.ceresoli@bootlin.com