[Cleanup] Remove npc_ai.cpp/npc_ai.cpp (#3081)

# Notes
- These files were unused.

zone/CMakeLists.txt

@@ -98,7 +98,6 @@ SET(zone_sources
     mob_movement_manager.cpp
     mob_info.cpp
     npc.cpp
-    npc_ai.cpp
     npc_scale_manager.cpp
     object.cpp
     oriented_bounding_box.cpp
@@ -236,7 +235,6 @@ SET(zone_headers
     mob.h
     mob_movement_manager.h
     npc.h
-    npc_ai.h
     npc_scale_manager.h
     object.h
     oriented_bounding_box.h