streams: Update error message returned for administrative access.

We have now decided to not continue with the stream administrator
concept as we are changing the permissions model to be based on
user groups as per #19525. So, this commit updates the error message
to "Must be an organization administrator".

zerver/lib/exceptions.py

@@ -241,17 +241,6 @@ class OrganizationOwnerRequired(JsonableError):
         return _("Must be an organization owner")
 
 
-class StreamAdministratorRequired(JsonableError):
-    code: ErrorCode = ErrorCode.UNAUTHORIZED_PRINCIPAL
-
-    def __init__(self) -> None:
-        pass
-
-    @staticmethod
-    def msg_format() -> str:
-        return _("Must be an organization or stream administrator")
-
-
 class AuthenticationFailedError(JsonableError):
     # Generic class for authentication failures
     code: ErrorCode = ErrorCode.AUTHENTICATION_FAILED