auxdisplay: img-ascii-lcd: Remove an unused field in struct img_ascii_lcd_ctx

Remove 'cfg' from struct img_ascii_lcd_ctx. It is unused since commit
7e76aece ("auxdisplay: Extract character line display core support")

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>