stripe: Remove invalid comment.

We use `error_description` in upgrade.ts to determine if the error was
related to customer's card. Doesn't seem like there is any harm
in doing so since we are explicitly handling "stripe.CardError" and
raising these errors with "card error" description.

corporate/lib/stripe.py

@@ -419,7 +419,6 @@ class BillingError(JsonableError):
     CONTACT_SUPPORT = gettext_lazy("Something went wrong. Please contact {email}.")
     TRY_RELOADING = gettext_lazy("Something went wrong. Please reload the page.")
 
-    # description is used only for tests
     def __init__(self, description: str, message: Optional[str] = None) -> None:
         self.error_description = description
         if message is None: