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: openwrt/openwrt#16505
Signed-off-by: Robert Marko <robimarko@gmail.com>