display: sdl: fix clear color for ARGB8888

Clear color for ARGB8888 was set to opaque white, which is an odd
choice.

This change makes the clear color "transparent".

Signed-off-by: Martin Stumpf <[email protected]>

Author: Finomnis

drivers/display/display_sdl.c

@@ -598,7 +598,6 @@ static int sdl_display_clear(const struct device *dev)
 	switch (disp_data->current_pixel_format) {
 	case PIXEL_FORMAT_ARGB_8888:
 		size = config->width * config->height * 4U;
-		bgcolor = 0xFFu;
 		break;
 	case PIXEL_FORMAT_RGB_888:
 		size = config->width * config->height * 3U;