🩹(backend) remove a non-existing field on BooleanValue

I used a LLM to review my code, it introduced an hallucinated field.
Remove it.

src/backend/meet/settings.py

@@ -459,13 +459,9 @@ class Base(Configuration):
 
     # Marketing and communication settings
     SIGNUP_NEW_USER_TO_MARKETING_EMAIL = values.BooleanValue(
-        False,
+        False,  # When enabled, new users are automatically added to mailing list.
         environ_name="SIGNUP_NEW_USER_TO_MARKETING_EMAIL",
         environ_prefix=None,
-        help_text=(
-            "When enabled, new users are automatically added to mailing list "
-            "for product updates, marketing communications, and customized emails. "
-        ),
     )
     MARKETING_SERVICE_CLASS = values.Value(
         "core.services.marketing_service.BrevoMarketingService",