pg_constraint.conincluding is useless

Hi

Already mentioned this in 
https://postgr.es/m/20180831205020.nxhw6ypysgshjtnl@alvherre.pgsql

While trying to add support for foreign keys to partitioned tables, I
noticed that commit 8224de4f42cc ("Indexes with INCLUDE columns and
their support in B-tree") added a column to pg_constraint that appears
to be there only to enable ruleutils.c to print out the list of columns
in a PRIMARY KEY or UNIQUE constraint that uses included columns.
However, this is pretty easy to obtain from pg_index.conkey instead, so
I claim that that column is useless.  In fact, here's a patch to remove
it.

This requires a catversion bump, for which it may seem a bit late;
however I think it's better to release pg11 without a useless catalog
column only to remove it in pg12 ...

Thoughts?

-- 
Álvaro Herrera