ide: remove CONFIG_BLK_DEV_IDEDOUBLER config option

Nowadays it is not worth having a separate config option for
Amiga IDE Doubler support so always include it (it still needs
to be explicitly enabled by module parameter).

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>