Prevent duplicate RTEPermissionInfo for plain-inheritance parents

Prevent duplicate RTEPermissionInfo for plain-inheritance parents

Currently, expand_single_inheritance_child() doesn't reset
perminfoindex in a plain-inheritance parent's child RTE, because
prior to 387f9ed0a0, the executor would use the first child RTE to
locate the parent's RTEPermissionInfo.  That in turn causes
add_rte_to_flat_rtable() to create an extra RTEPermissionInfo
belonging to the parent's child RTE with the same content as the one
belonging to the parent's original ("root") RTE.

In 387f9ed0a0, we changed things so that the executor can now use the
parent's "root" RTE for locating its RTEPermissionInfo instead of the
child RTE, so the latter's perminfoindex need not be set anymore, so
make it so.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/839708.1698174464@sss.pgh.pa.us
Backpatch-through: 16