more sortloot revamp fallout

more sortloot revamp fallout

getobj() was caching 'invent' in 'firstobj', dating from the days
of the !GOLDOBJ configuration, and sortloot() could change the value
of invent, making firstobj end up pointing somewhere into the midst
of the inventory list.  So collecting letters of applicable items
could miss things (typically right after restoring a saved game).
Repeating the command would operate on already sorted invent, making
firstobj remain valid and things mysteriously reappear after having
been missed before.

Just get rid of 'firstobj' since it's no longer useful.