CallWindow: don't set the title for multiple participants
CallWindow: don't set the title for multiple participants

We don't support that yet, the string is wrong, and
priv->contact can't be NULL anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=655478

GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets