[FIX] test_mail, various: remove extra gateway initialize

When inheriting from the mail test class gateway initialization is done
automatically. No need to call it again.

Task-3453577 (TestMail: Update Alias/Gateway tests for MC)
Prepares Task-36879 (Mail: Support MultiCompany Aliases)

Part-of: odoo/odoo#135055

addons/mail_group/tests/common.py

@@ -55,8 +55,6 @@ class TestMailListCommon(MailCommon):
         })
         cls.test_group_valid_members = cls.test_group_member_1 + cls.test_group_member_2 + cls.test_group_member_4_emp
 
-        cls._init_mail_gateway()
-
         # Create some messages
         cls.test_group_msg_1_pending = cls.env['mail.group.message'].create({
             'subject': 'Test message pending',

addons/test_crm_full/tests/common.py

@@ -13,7 +13,6 @@ class TestCrmFullCommon(TestCrmCommon, MockIAPReveal, MockVisitor):
     @classmethod
     def setUpClass(cls):
         super(TestCrmFullCommon, cls).setUpClass()
-        cls._init_mail_gateway()
         cls._activate_multi_company()
 
         # Context data: dates

addons/test_event_full/tests/common.py

@@ -15,7 +15,6 @@ class TestEventFullCommon(EventCrmCase, TestSalesCommon, MockVisitor):
     @classmethod
     def setUpClass(cls):
         super(TestEventFullCommon, cls).setUpClass()
-        cls._init_mail_gateway()
 
         # Context data: dates
         # ------------------------------------------------------------