[FIX] website_forum: remove not working channel_ids given to composer
[FIX] website_forum: remove not working channel_ids given to composer

Mail composer does not support channels. Indeed as it can be used to post
a message or send an email it only supports partners. Previously field
channel_ids given to the composer was used only in website_forum. However
it is not used and therefore simplest way of solving it is to remove it.

Anyway allowing to have channels followers of forum tags and being notified
of new publications is probably more spammy than really necessary. Especially
that it never worked.

Commit linked to task ID 1907153 and PR #28464
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo