[FIX] mail: counter on mailing channel when user handle notification with emails

Before this commit:

Message from mailing channel make a notification in Odoo for users
with notification "Handled by Email" when user is not log-in. this is happening
because mailing channel is marked as seen from
"messaging_notification_handler.js" which is not executed when user is not
log-in and receive message so message counter is increase when user log-in.

After this commit:

Hide the counter for mailing channel when user handles notification with emails.

Task-2541660

closes odoo/odoo#82015

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>