ar71xx-generic: Generate OpenMesh sysupgrades in OpenWrt

The workaround to generate sysupgrade images for OpenMesh devices in gluon
is replaced in LEDE/OpenWrt by a special patch. It is therefore better to
drop the workaround and use the upstream version.

Reported-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>