Socket: Don't add instances twice in cls._instances.

The connect() method already adds it, so it was in the list twice
(added both by __init__() and connect()).

This caused _select() to call _read() twice on the same instance,
except there is usually nothing to read on the second call,
so it blocks for up to conf.supybot.drivers.poll().

src/drivers/Socket.py

@@ -65,7 +65,6 @@ class SocketDriver(drivers.IrcDriver, drivers.ServersMixin):
     _instances = []
     _selecting = threading.Lock()
     def __init__(self, irc):
-        self._instances.append(self)
         assert irc is not None
         self.irc = irc
         drivers.IrcDriver.__init__(self, irc)