mint-arena: Don't include menudef.h in ai_chat.c

I commented it out in Turtle Arena. Removed and still compiles, so went ahead and removed it.
mint-arena: Don't hardcode the list of near by items for bot chat

Use bg_itemlist and specifically ignore some item types instead of duplicating the names.

Obelisks are not in the item list, so name must still be hardcoded for near by items.
mint-arena: Use wrappers for accessing bg_itemlist

Turtle Arena uses BG_NumItems, BG_ItemNumForItem, and BG_ItemForItemNum to access item info.
Adding these to mint-arena makes merging between mint-arena and Turtle Arena easier for me.