prometheus-node-exporter-lua: drop bmx6 package

prometheus-node-exporter-lua: drop bmx6 package

In the OpenWrt routing feed, package bmx6 and luci-app-bmx6 were removed because the LuCI app was vulnerable to several CVEs, as found by dependabot. It has been reporting it for a few months and has even created an issue. These two packages are not maintained in OpenWrt as well in upstream.

Users should switch to the bmx7 package.

Fixes: 9fb9d9343ea27d6dbb5008ece10c0c843dd2c781 ("bmx6: drop package") in the routing feed
Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>