npc: allied NPCs aren't shopkeepers

[andrei8l]

Summary

None

Purpose of change

Allied "shopkeepers" improperly run some shopkeeper code since #62009

Describe the solution

Make sure allied NPCs aren't shopkeepers

Describe alternatives you've considered

N/A

Testing

1. Spawn an NPC and change its class to NC_HUNTER, which has a shopkeeper_item_group defined
2. Ally the NPC and open the trade ui
3. Verify (lack of) fallback zones

Additional context

Fixes the issue from this reddit thread
I was pretty sloppy with #62009 and I'm sorry

[Fris0uman]

There are no cases where we want an ally NPC to behave like a shopkeeper?

[andrei8l]

There are no cases where we want an ally NPC to behave like a shopkeeper?

I don't think so, as it'd be pretty hard to justify them restocking out of the ether while following the player around. It's also (currently) assumed elsewhere in code that allied NPCs always trade for free.