npctalk: fix remove_npc

[andrei8l]

Summary

None

Purpose of change

remove_npc effect doesn't work because:

1. it's not even used
2. it only checks loaded NPCs
3. it filters them by a temp variable that isn't de/serialized

the result is that the random encounter NPCs from #56629 and #56940 never despawn

Fixes #57549

Describe the solution

1. Actually use this function...
2. Check all NPCs on the overmap
3. Filter them by at least one of: class, talk topic, or unique_id
4. Change the add eoc type to OM_MOVE so it doesn't spawn spurious NPCs if there's no evac_center on the overmap

Describe alternatives you've considered

De/serializing npc.idz instead of 3: this would work, but not fix the issue in existing saves obsolete

Testing

1. Load this pruned save Kenansville.tar.gz (EDIT: this save is missing the unique_id for John Bailey and github isn't letting me upload an update atm)
2. Observe squatting visitors
3. Teleport outside of reality bubble and wait 6 hours
4. Teleport back and see all the (existing) visitors have despawned
5. Confirm that visitors still spawn by waiting and moving near the evac_center (crank up "one_in_chance" to 1 for this)

Try the repro steps from #57549 too

Additional context

N/A

[andrei8l]

Updated to use unique_id for John Bailey and also fixed visitors spawning near the player if there's no evac center around.