Simplify more checks related to set-returning functions

This makes more consistent the SRF-related checks in the area of
PL/pgSQL, PL/Perl, PL/Tcl, pageinspect and some of the JSON worker
functions, making it easier to grep for the same error patterns through
the code, reducing a bit the translation work.

It is worth noting that each_worker_jsonb()/each_worker() in jsonfuncs.c
and pageinspect's brin_page_items() were doing a check on expectedDesc
that is not required as they fetch their tuple descriptor directly from
get_call_result_type().  This looks like a set of copy-paste errors that
have spread over the years.

This commit is a continuation of the changes begun in 07daca5, for any
remaining code paths on sight.  Like fcc2817, this makes the code more
consistent, easing the integration of a larger patch that will refactor
the way tuplestores are created and checked in a good portion of the
set-returning functions present in core.

I have worked my way through the changes of this patch by myself, and
Ranier has proposed the same changes in a different thread in parallel,
though there were some inconsistencies related in expectedDesc in what
was proposed by him.

Author: Michael Paquier, Ranier Vilela
Discussion: https://postgr.es/m/CAAKRu_azyd1Z3W_r7Ou4sorTjRCs+PxeHw1CWJeXKofkE6TuZg@mail.gmail.com
Discussion: https://postgr.es/m/CAEudQApm=AFuJjEHLBjBcJbxcw4pBMwg2sHwXyCXYcbBOj3hpg@mail.gmail.com

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