ipq807x: delete unused legacy eMMC sysupgrade code

ipq807x: delete unused legacy eMMC sysupgrade code

All ipq807x devices that were using the legacy 'mmc_do_upgrade' eMMC
sysupgrade code were ported to the replacement 'emmc_do_upgrade' code.

Signed-off-by: Rodrigo Balerdi <lanchon@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16505
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit 491121288eaa9d20f3db8215a4790e8f0ee1f8ca)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>