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
(cherry picked from commit 4d90b79704efba57ab158c47ae6936ce4c1ef185)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>