Fix import in code generation api template (#891)

Author: wu-clan

backend/plugin/code_generator/templates/python/api.jinja

@@ -15,7 +15,6 @@ from backend.common.security.jwt import DependsJwtAuth
 from backend.common.security.permission import RequestPermission
 from backend.common.security.rbac import DependsRBAC
 from backend.database.db import CurrentSession, CurrentSessionTransaction
-from backend.database.db import CurrentSession
 
 router = APIRouter()