ramips: mt7621: Edgerouter-X drop factory image

ramips: mt7621: Edgerouter-X drop factory image

Edgerouter-X factory images have not built automatically since 19.x due
to images being over 3MB. While it was possible to build custom images
with very stripped down config, this is no longer possible with the size
increases of linux 6.1 and 6.6.

Drop code for generation of factory images, if some dev later wishes to
try custom images they can revert this commit.

Signed-off-by: Tim Lunn <tim@feathertop.org>
Link: https://github.com/openwrt/openwrt/pull/15194
Signed-off-by: Petr Štetiar <ynezz@true.cz>