[REM] hr: remove unimported file

Leftover of 78ac6de5 code about mail gateway was moved to
mail_alias.py but mail_thread.py file was not removed.

addons/hr/models/mail_thread.py

-# -*- coding: utf-8 -*-
-# Part of Odoo. See LICENSE file for full copyright and licensing details.
-
-from odoo import models, tools
-from odoo.tools import email_split
-
-
-class MailAlias(models.AbstractModel):
-    _inherit = 'mail.alias.mixin'
-
-    def _alias_check_contact(self, message, message_dict, alias):
-        if alias.alias_contact == 'employees' and self.ids:
-            email_from = tools.decode_message_header(message, 'From')
-            email_address = email_split(email_from)[0]
-            employee = self.env['hr.employee'].search([('work_email', 'ilike', email_address)], limit=1)
-            if not employee:
-                employee = self.env['hr.employee'].search([('user_id.email', 'ilike', email_address)], limit=1)
-            if not employee:
-                return {
-                    'error_message': 'restricted to employees',
-                    'error_template': self.env.ref('hr.mail_template_data_unknown_employee_email_address').body_html,
-                }
-            return True
-        return super(MailAlias, self)._alias_check_contact(message, message_dict, alias)