Before this commit odoobot was appearing twice in mention
suggestions when talking to odoobot.
The initial need to be able to ping odoobot from everywhere led to the
override of channel_fetch_listeners. This is not usefull anymore.
This fix simply removes this override to avoid this problem since we
don't have a real use case where we need to ping odoobot in another
channel.
Task 1907142
closesodoo/odoo#28791
XavierDo
136fee6fcf