Renames flatten_list proc to assoc_to_values (#93453)

## About The Pull Request

`/proc/flatten_list()` -> `/proc/assoc_to_values()`, also changes the
code doc to mirror the language of `assoc_to_keys()`'s code doc

## Why It's Good For The Game

Having a proc called `assoc_to_keys` that takes an associative list and
returns a list of the keys, and also having a proc that takes an
associative list and returns a list of the values, and the latter not
being called `assoc_to_values` is very funny

## Changelog
🆑
code: renamed flatten_list proc to assoc_to_values
/🆑

---------

Co-authored-by: Ghom <42542238+Ghommie@users.noreply.github.com>

code/modules/admin/smites/retcon.dm

@@ -27,7 +27,7 @@
 
 /datum/smite/retcon/proc/delete_bank_account(mob/living/target)
 	var/name = target.real_name
-	var/account_list = flatten_list(SSeconomy.bank_accounts_by_id)
+	var/account_list = assoc_to_values(SSeconomy.bank_accounts_by_id)
 	for(var/datum/bank_account/account in account_list)
 		if(account.account_holder == name)
 			qdel(account)

code/modules/admin/verbs/hiddenprints.dm

@@ -8,7 +8,7 @@
 	if(!islist(victim_hiddenprints))
 		victim_hiddenprints = list()
 
-	var/list/hiddenprints = flatten_list(victim_hiddenprints)
+	var/list/hiddenprints = assoc_to_values(victim_hiddenprints)
 	list_clear_nulls(hiddenprints)
 
 	if(!length(hiddenprints))