dubious warning: FORMAT JSON has no effect for json and jsonb types

This warning comes from parse_expr.c transformJsonValueExpr() and is 
triggered for example by the following test case:

SELECT JSON_OBJECT('foo': NULL::json FORMAT JSON);
WARNING:  FORMAT JSON has no effect for json and jsonb types

But I don't see anything in the SQL standard that would require this 
warning.  It seems pretty clear that FORMAT JSON in this case is 
implicit and otherwise without effect.

Also, we don't have that warning in the output case (RETURNING json 
FORMAT JSON).

Anyone remember why this is here?  Should we remove it?