Remove the Valhallists for 0.I experimental

[MNG-cataclysm]

Summary

Content "Remove final mentions of Valhallists."

Purpose of change

The Valhallists were added and then obsoleted in 0.G, and their files have remained dormant for a long time. They needed a lot of work to fit into the base game with the state they are in, as it teetered just over the edge of what was reasonably realistic, despite two attempts to rework them. It's a project I no longer have the time for, and thus be able to do, as I've gathered other project commitments with different NPC factions.

Describe the solution

Remove the remaining files for the Valhallist NPCs, including missions, snippets, and the vehicle purchasing system. Also delete the faction ID and epilogue snippets from their relevant files. Everything else has already been obsoleted, and #71666 removes that as part of it's content

Describe alternatives you've considered

Reviving and fixing the Valhallists, which would require lots of design and discussion with head developers to create a lore-friendly faction, which would probably look significantly different from what is already present. This is also a lot of work that I don't have the time for at the moment.

Testing

N/A, none of this has been reflecting in game since 2023.

Additional context

@Maleclypse I've got the files in reserve for your use, since I recall you'd mentioned wanting to do something with them for Xedra Evolved.