drm/panel: sitronix-st7789v: remove unused constants

ST7789V_COLMOD_RGB_FMT_18BITS and ST7789V_COLMOD_CTRL_FMT_18BITS
are unused in favour of MIPI_DCS_PIXEL_FMT_18BIT, remove them.

Reviewed-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230714013756.1546769-5-sre@kernel.org