Having those fields as translatable lead people to think those will be
translated into recipient's language. This is incorrect as translations
in Odoo are based on current user, not about any recipient who would
see the field value.
In this commit we therefore remove translation capabilities for
subject and preview. Indeed mailings currently offer no multi languages
support. Instead people should create separate mailings targetting
some specific recipients, based on lang or country for example.
Task ID-2535807
closesodoo/odoo#71093
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>