Remove broken Assertions that failed if a statement executed in PL/pgSQL is

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.