🎨 [#4650] Revert generic test changes

The test module is aimed at the generic registration mechanism and
just 'happens' to use the email plugin. We do not need to repeat all
the various cases/failure modes that are already covered by the
plugin unit tests.

src/openforms/registrations/tests/test_registration_hook.py

@@ -312,7 +312,7 @@ def test_registration_backend_invalid_options(self):
             completed=True,
             form__registration_backend="email",
             form__registration_backend_options={},
-        )  # Missing "to_emails" and "to_emails_from_variable" option
+        )  # Missing "to_emails" option
 
         with (
             self.subTest("On completion - does NOT raise"),
@@ -334,48 +334,6 @@ def test_registration_backend_invalid_options(self):
         ):
             register_submission(submission.id, PostSubmissionEvents.on_retry)
 
-    def test_email_registration_backend_with_to_emails(self):
-        submission = SubmissionFactory.create(
-            completed=True,
-            form__registration_backend="email",
-            form__registration_backend_options={"to_emails": ["[email protected]"]},
-        )
-
-        with (
-            self.subTest("On completion - logs as successful"),
-            self.assertLogs(level="INFO") as logs,
-        ):
-            register_submission(submission.id, PostSubmissionEvents.on_completion)
-
-            submission.refresh_from_db()
-            self.assertIn(
-                "Registration using plugin '%r' for submission '%s' succeeded",
-                logs.records[-1].msg,
-            )
-            self.assertFalse(submission.needs_on_completion_retry)
-
-    def test_email_registration_backend_with_to_emails_from_variable(self):
-        submission = SubmissionFactory.create(
-            completed=True,
-            form__registration_backend="email",
-            form__registration_backend_options={
-                "to_emails_from_variable": "email_example_variable"
-            },
-        )
-
-        with (
-            self.subTest("On completion - logs as successful"),
-            self.assertLogs(level="INFO") as logs,
-        ):
-            register_submission(submission.id, PostSubmissionEvents.on_completion)
-
-            submission.refresh_from_db()
-            self.assertIn(
-                "Registration using plugin '%r' for submission '%s' succeeded",
-                logs.records[-1].msg,
-            )
-            self.assertFalse(submission.needs_on_completion_retry)
-
     def test_calling_registration_task_with_serialized_args(self):
         submission = SubmissionFactory.create(
             completed=True,