test_auth_backends: Remove unused self.user_profile.backend.

This was added in d43b031a32 and was
unused when it was added. This is an error that we want to remove.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

zerver/tests/test_auth_backends.py

@@ -880,7 +880,6 @@ class SocialAuthBase(DesktopFlowTestingLib, ZulipTestCase, ABC):
         self.name = self.user_profile.full_name
         self.backend = self.BACKEND_CLASS
         self.backend.strategy = DjangoStrategy(storage=BaseDjangoStorage())
-        self.user_profile.backend = self.backend
         self.logger_string = f"zulip.auth.{self.backend.name}"
 
         # This is a workaround for the fact that Python social auth