Instead of trying (and failing) to allow <<label>> at the end of a DECLARE

Instead of trying (and failing) to allow <<label>> at the end of a DECLARE
section, throw an error message saying explicitly that the label must go
before DECLARE.  Per investigation of a recent pgsql-novice question,
this code did not work as intended in any modern PG version, maybe not ever.
Allowing such a thing would only create ambiguity anyway, so it seems better
to remove it than fix it.