imx8image: Remove unused cont_img_count variable

With clang-15, it is now reported that cont_img_count is unused. This is
true as the code will increment / reset this counter, but never
functionally use it. Remove it.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>