remove presence handling

slack's irc server handles presence using native irc AWAY commands,
but irssi has a bug in sending AWAY commands that un-set away status.

remove our unnecessary presence handling; this script works around the
bug in irssi:

  https://github.com/irssi/irssi/issues/141#issuecomment-55063025
Merge pull request #8 from jwm/jwm_remove_away_handling

remove presence handling