Removes unused code from stock market and an unused market define. (#83872)

## About The Pull Request

This PR removes a completely unused file, which added
`/datum/supply_pack`s originally containing the materials for the
mineral stock market contents, plus a define, MARKET_QUANTITY_HELPERS.
Neither of these wound up being used in the final revision of the
galactic mineral market, and I stumbled upon these to my horror that
they're still in the code and completely unused as a result. Yikes!

## Why It's Good For The Game

Removes unused code that is both not being utilized by the mineral
market, but in fact was superseded by the current implementation during
development and managed to slip it's way through review.

## Changelog

No front facing changes, as the code was unused.

code/modules/cargo/packs/stock_market_items.dm

@@ -1,35 +0,0 @@
-/**
- * todo: make this a supply_pack/custom. Drop pod? ohoho yes. Would be VERY fun.
- */
-/datum/supply_pack/market_materials
-	name = "A Single Sheet of Bananium"
-	desc = "Going market price for this kind of sheet, by Australicus Industrial Mining."
-	cost = CARGO_CRATE_VALUE * 2
-	crate_name = "mineral stock sheet crate"
-	group = "Canisters & Materials"
-	/// What material we are trying to buy sheets of?
-	var/datum/material/material
-	/// How many sheets of the material we are trying to buy at once?
-	var/amount
-
-/datum/supply_pack/market_materials/get_cost()
-	for(var/datum/material/mat as anything in SSstock_market.materials_prices)
-		if(material == mat)
-			return SSstock_market.materials_prices[mat] * amount
-
-/datum/supply_pack/market_materials/fill(obj/structure/closet/crate/C)
-	. = ..()
-	new material.sheet_type(C, amount)
-
-/datum/supply_pack/market_materials/iron
-	name = "Iron Sheets"
-	crate_name = "iron stock crate"
-	material = /datum/material/iron
-MARKET_QUANTITY_HELPERS(/datum/supply_pack/market_materials/iron)
-
-
-/datum/supply_pack/market_materials/gold
-	name = "Gold Sheets"
-	crate_name = "gold stock crate"
-	material = /datum/material/gold
-MARKET_QUANTITY_HELPERS(/datum/supply_pack/market_materials/gold)