[arch-releng] How about removing support for including [core]

hi, as it seems we will only provide netinstall isos anyway, we probably can simplify the releng part of archiso a bit by. Maybe we can also remove the need to create the arch specific isos and just create one single dual iso. And finally we could also the no longer useful dual-netinstall prefix from the iso names. Any thoughts about this? Greetings, Pierre -- Pierre Schmitz, https://pierre-schmitz.com