uip/inc.c: Delete POP3's -pack option for msh users.

uip/inc.c: Delete POP3's -pack option for msh users.

inc(1)'s -pack option, only available with POP3 access, stored the
emails in a spool file instead of a folder.  The man page says this was
intended for msh(1) users, but msh was deleted in e6917522.  The spool
file was always in packf(1)'s MMDF format, no option of -mbox, and was a
rare producer of `map' files that indexed the spool file, again intended
for msh.  Removing -pack makes deletion of map files elsewhere in the
code easier.