procd: make nand_do_platform_check check image only

Recently all targets have been updated to call nand_do_upgrade directly.
It's not needed anymore to leave a magic /tmp/sysupgrade-nand-path mark
during image check.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: John Crispin <blogic@openwrt.org>

SVN-Revision: 47079