Remove unused argument "txn" in maybe_send_schema().
Commit
464824323e added the argument "txn" into maybe_send_schema()
though it was not used.
Author: Hou Zhijie
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/OS0PR01MB5716146E43928FB92D45D29794EC9@OS0PR01MB5716.jpnprd01.prod.outlook.com