[IMP] base: Remove useless ACL rules

Now that portal users don't have access to the backend, it doesn't make
sense to give the read access on attachments, as this is the role of
the specific controllers to provide this access according to the business
logic.

For the record, these rules have been introduced at:
https://github.com/odoo/odoo/commit/61065b6d04248aa496765e1035c4c90cbdc38de7
https://github.com/odoo/odoo/commit/f3fa266d115ac9d4723164af10f8dee40821b290



closes odoo/odoo#32134

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>