Remove incorrect assertion for INSERT in logical replication's publisher

Remove incorrect assertion for INSERT in logical replication's publisher

On the publisher, it was assumed that an INSERT change cannot happen for
a relation with no replica identity.  However this is true only for a
change that needs references to old rows, aka UPDATE or DELETE, so
trying to use logical replication with a relation that has no replica
identity led to an assertion failure in the publisher when issuing an
INSERT.  This commit removes the incorrect assertion, and adds more
regression tests to provide coverage for relations without replica
identity.

Reported-by: Neha Sharma
Author: Dilip Kumar, Michael Paquier
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/CANiYTQsL1Hb8_Km08qd32svrqNumXLJeoGo014O7VZymgOhZEA@mail.gmail.com
Backpatch-through: 10