NPC: Cleaned up debugging logs.

Standarized logs to have different levels.
Show by default only INFO, ERROR log messages.
Dialogues: Standarized function for NPC chat messages.
Removed trader.lua and trader_test.lua which are part of old 'mobs_npc' mod.

dialogue.lua

@@ -296,7 +296,7 @@ function npc.dialogue.process_dialogue(self, dialogue, player_name)
 
 	-- Send dialogue line
 	if dialogue.text then
-		minetest.chat_send_player(player_name, self.npc_name..": "..dialogue.text)
+		npc.chat(self.npc_name, player_name, dialogue.text)
 	end
 
   -- Check if dialogue has responses. If it doesn't, unlock the actions