configs/olimex_imx233_olinuxino: remove useless section from readme.txt

configs/olimex_imx233_olinuxino: remove useless section from readme.txt

After the conversion to the common genimage scripts, the uImage.imx23-olinuxino
file no longer exists.

Instead of updating the section, let's simply remove it as the more
standard method of using sdcard.img is already explained.

This also makes the readme more consistent with the other i.MX readme files.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>