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>
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Cc: Peng Fan <peng.fan@nxp.com>
Cc: Mikhail Ilin <ilin.mikhail.ol@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: "NXP i.MX U-Boot Team" <uboot-imx@nxp.com>