[FIX] mail: deletable attachments in attachment box

Revision on https://github.com/odoo/odoo/commit/e7aff236b8073b952a0e554d9897a8f88d5c4c91

Commit above remove cross icon on attachments from messages that are
not in the chatter, since this only makes sense in the chatter.

However, it also removed by mistake the possibility to delete
attachments from the attachment box in the chatter.

This happens because the template of attachments uses a newly
introduced contextual boolean `isDeletable`, and this is false by
default due to undefined being falsy.

This commit fixes the issue by explicitly enabling deletion of
attachments from the attachment box.

Task-Id 2081987

closes #38003