When making a poll, an event is registered on each channel. Once
a notification is sent on a channel, only the events of the
notified channel are removed. A pointer to the event is kept
in all other channel the user subscribes to until those channel
are notified.
Since a user usually has many mail.channels but only a few active ones,
a new event and a bunch of pointers are created at each poll and
never removed.
This fix simply remove the pointers to the current event at the
end of a poll.
closesodoo/odoo#31215
Signed-off-by: Toufik Benjaa (tbe) <tbe@odoo.com>