Commit 2ec993a7, which added triggers on views, modified the rewriter
to add dummy entries like "SET x = x" for all columns that weren't
actually being updated by the user in any UPDATE directed at a view.
That was needed at the time to produce a complete "NEW" row to pass
to the trigger.  Later it was found to cause problems for ordinary
updatable views, so commit cab5dc5d restricted it to happen only for
trigger-updatable views.  But in the wake of commit 86dc9005, we
really don't need it at all.  nodeModifyTable.c populates the trigger
"OLD" row from the whole-row variable that is generated for the view,
and then it computes the "NEW" row using that old row and the UPDATE
targetlist.  So there is no need for the UPDATE tlist to have dummy
entries, any more than it needs them for regular tables or other
types of views.

(The comments for rewriteTargetListIU suggest that we must do this
for correct expansion of NEW references in rules, but I now think
that that was just lazy comment editing in 2ec993a7.  If we didn't
need it for rules on views before there were triggers, we don't need
it after that.)

This essentially propagates 86dc9005's decision that we don't need
dummy column updates into the view case.  Aside from making the
different cases more uniform and hence possibly forestalling future
bugs, it ought to save a little bit of rewriter/planner effort.

Discussion: https://postgr.es/m/2181213.1619397634@sss.pgh.pa.us