Remove unproduceable 'mail' armor from raws.

Mail shirts and other mail items are still available, as these both
have reactions and are used by invaders. Only ITEM_ARMOR_MAIL was
removed, as it could only be obtained from a mood (although could
then be copied by the inventor).

See http://www.bay12forums.com/smf/index.php?topic=125628.msg4774188#msg4774188