(buy, check_market, trad, check_trade): Remove the following virtually

(buy, check_market, trad, check_trade): Remove the following virtually
undocumented feature: When a buyer didn't have enough money at the
time of the trade, the seller *automatically* granted a high-interest,
medium term loan for up to 90% of the price.  This is just a silly
trap for unwary sellers and buyers alike.
(N_FIN_TROUBLE, N_CREDIT_JUNK): Unused, remove.
(rpt): Update accordingly.