Remove option TRADESHIPS, customize table ship-chr instead

Remove option TRADESHIPS, customize table ship-chr instead

Trade ships are now enabled when a ship type with capability trade
exists.  No such type exists by default; to enable trade ships,
deities have to customize table ship-chr.

Before, trade ship types were ignored when option TRADESHIPS was
disabled.  Except for xdump ship-chr, which happily dumped unusable
trade ship types.