Remove two_phase variable from CreateReplicationSlotCmd struct.

Remove two_phase variable from CreateReplicationSlotCmd struct.

Commit 19890a064e added the option to enable two_phase commits via
pg_create_logical_replication_slot but didn't extend the support of same
in replication protocol. However, by mistake, it added the two_phase
variable in CreateReplicationSlotCmd which is required only when we extend
the replication protocol.

Reported-by: Jeff Davis
Author: Ajin Cherian
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/64b9f783c6e125f18f88fbc0c0234e34e71d8639.camel@j-davis.com