[REM] website_sale_coupon: Fields and file not used anymore

Since this commit https://github.com/odoo/enterprise/commit/87d542, these fields are not used anymore.
It needed to be removed in master.

models/__init__.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
-from . import sale_coupon_rule
 from . import sale_order
 from . import website

models/sale_coupon_rule.py

@@ -1,11 +0,0 @@
-# -*- coding: utf-8 -*-
-
-from odoo import models, fields
-
-
-class SaleCouponRule(models.Model):
-    _inherit = "sale.coupon.rule"
-
-    # TODO Remove in master, useless now
-    is_public_included = fields.Boolean(string="Include Public User",
-        help="Is the public user included into the set of autorized customers", default=True, deprecated=True)