pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/30168be8f75b95183abccf48f0da7a64a0cfbd9f

Modified Files
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.../plpgsql/src/expected/plpgsql_transaction.out   |  2 +-
src/pl/plpgsql/src/pl_exec.c                       | 37 ----------------------
src/pl/plpgsql/src/pl_funcs.c                      | 23 --------------
src/pl/plpgsql/src/pl_gram.y                       | 36 +--------------------
src/pl/plpgsql/src/pl_unreserved_kwlist.h          |  2 --
src/pl/plpgsql/src/plpgsql.h                       | 14 +-------
6 files changed, 3 insertions(+), 111 deletions(-)