[MDC-643] simetbox-openwrt-availability-config: remove ifupdown legacy

[MDC-643] simetbox-openwrt-availability-config: remove ifupdown legacy

Removes the remants of the sendifupdown legacy test, since it wears out
FLASH by writing to /etc/config, and it has been superseded by "inetup".

* Requires simetbox-openwrt-base v105