[REF] mass_mailing, project: remove deprecated '_period_number' attribute

This parameter declared on mailing and project classes is not used anymore.
Let us remove it to clean model definitions.

closes odoo/odoo#39401

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mass_mailing/models/mailing.py

@@ -40,8 +40,6 @@ class MassMailing(models.Model):
     _name = 'mailing.mailing'
     _description = 'Mass Mailing'
     _inherit = ['mail.thread', 'mail.activity.mixin', 'mail.render.mixin']
-    # number of periods for tracking mail_mail statistics
-    _period_number = 6
     _order = 'sent_date DESC'
     _inherits = {'utm.source': 'source_id'}
     _rec_name = "subject"

addons/project/models/project.py

@@ -66,7 +66,6 @@ class Project(models.Model):
     _description = "Project"
     _inherit = ['portal.mixin', 'mail.alias.mixin', 'mail.thread', 'rating.parent.mixin']
     _order = "sequence, name, id"
-    _period_number = 5
     _rating_satisfaction_days = False  # takes all existing ratings
     _check_company_auto = True