[FIX] mail: remove `mail_followers` widget support claim
[FIX] mail: remove `mail_followers` widget support claim

This widget claims to support one2many fields but it doesn't work in most cases.
This was triggering warnings when this widget was set on another field or in another
view (i.e. in Studio).

opw-1905803

closes odoo/odoo#28413
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo