[FIX] mail: allow user to delete other's attachments again

Before this PR, only author of the message and admin could delete attachments
inside the chatter.
This behavior is now restricted to the channels and allow users to delete
attachments inside the chatter.

task-2753295

closes odoo/odoo#85469

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>