Cargo category cleanup (#75374)

## About The Pull Request

I accidentally discovered that when #55504 removed cargo export
categories being a thing during export value evaluation, in two
instances, they forgot to remove passing over the now defunct export
categories, causing it to be passed in as the elasticity value, and by
sheer coincidence, this was not causing problems (due to being overriden
by a named argument in the pirates case, and cargo_exports being
evaluated as true in another).

This PR fixes the arguments, preventing possible bugs in the future.
This also removes the code that set if the cargo shuttle could sell
contraband and emagged items, as that is no longer a thing. I talked
with @ArcaneMusic about this, and they agreed, albeit with the caveat
that if someone finds a good use case for this feature, it could be
saved.

This PR also autodocs several export related files, and cleans up
various single and two letter vars.

## Why It's Good For The Game

Cleaner code, unused code removed.

## Changelog

🆑
code: cleaned up cargo export code a bit
/🆑

code/modules/modular_computers/file_system/programs/budgetordering.dm

@@ -33,9 +33,6 @@
 	///The account this console processes and displays. Independent from the account the shuttle processes.
 	var/cargo_account = ACCOUNT_CAR
 
-/datum/computer_file/program/budgetorders/proc/get_export_categories()
-	. = EXPORT_CARGO
-
 /datum/computer_file/program/budgetorders/proc/is_visible_pack(mob/user, paccess_to_check, list/access, contraband)
 	if(issilicon(user)) //Borgs can't buy things.
 		return FALSE
@@ -169,7 +166,6 @@
 				computer.say(blockade_warning)
 				return
 			if(SSshuttle.supply.getDockedId() == docking_home)
-				SSshuttle.supply.export_categories = get_export_categories()
 				SSshuttle.moveShuttle(cargo_shuttle, docking_away, TRUE)
 				computer.say("The supply shuttle is departing.")
 				usr.investigate_log("sent the supply shuttle away.", INVESTIGATE_CARGO)