Remove plpgsql's special-case code paths for SET/RESET.

Remove plpgsql's special-case code paths for SET/RESET.

In the wake of 84f5c2908, it's no longer necessary for plpgsql to
handle SET/RESET specially.  The point of that was just to avoid
taking a new transaction snapshot prematurely, which the regular code
path through _SPI_execute_plan() now does just fine (in fact better,
since it now does the right thing for LOCK too).  Hence, rip out a
few lines of code, going back to the old way of treating SET/RESET
as a generic SQL command.  This essentially reverts all but the
test cases from b981275b6.

Discussion: https://postgr.es/m/15990-eee2ac466b11293d@postgresql.org