pgsql: Back out the session_start and session_end hooks feature.

Back out the session_start and session_end hooks feature.

It's become apparent during testing that there are problems with at
least the testing regime. I don't think we should have it without a
working test regime, and the difficulties might indicate implementation
problems anyway, so I'm backing out the whole thing until that's sorted
out.

This reverts commits 7459484 9989f92 cd8ce3a

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/98d54bb7790d5fb0a77173d5e5e3c655901b472c

Modified Files
--------------
src/backend/tcop/postgres.c                        |   6 -
src/backend/utils/init/postinit.c                  |   6 -
src/include/tcop/tcopprot.h                        |   7 --
src/test/modules/Makefile                          |   1 -
src/test/modules/test_session_hooks/.gitignore     |   4 -
src/test/modules/test_session_hooks/Makefile       |  25 ----
src/test/modules/test_session_hooks/README         |   2 -
.../expected/test_session_hooks.out                |  31 -----
.../modules/test_session_hooks/session_hooks.conf  |   2 -
.../test_session_hooks/sql/test_session_hooks.sql  |  12 --
.../test_session_hooks/test_session_hooks--1.0.sql |   4 -
.../test_session_hooks/test_session_hooks.c        | 134 ---------------------
.../test_session_hooks/test_session_hooks.control  |   3 -
src/tools/msvc/vcregress.pl                        |   2 -
14 files changed, 239 deletions(-)