pgsql: Remove broken Assertions that failed if a statement executed in

Log Message:
-----------
Remove broken Assertions that failed if a statement executed in PL/pgSQL is
rewritten into another kind of statement, for example if an INSERT is
rewritten into an UPDATE.

Back-patch to 8.3 and 8.2. For HEAD, Tom suggested inventing a new
SPI_OK_REWRITTEN return code, but that's not a backportable solution. I'll
do that as a separate patch, this patch will do as a stopgap measure for HEAD
too in the meanwhile.

Tags:
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REL8_3_STABLE

Modified Files:
--------------
    pgsql/src/pl/plpgsql/src:
        pl_exec.c (r1.202.2.2 -> r1.202.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c?r1=1.202.2.2&r2=1.202.2.3)