pgsql: Revert "postgres_fdw: Inherit the local transaction's access/def

Revert "postgres_fdw: Inherit the local transaction's access/deferrable modes."

We concluded that commit e5a3c9d9b is a feature rather than a fix; since
it was added after feature freeze, revert it.

Reported-by: Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
Reported-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reported-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Discussion: https://postgr.es/m/ed2296f1-1a6b-4932-b870-5bb18c2591ae%40oss.nttdata.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7d4667c6203d0174903a53de9d1730fbec26576b

Modified Files
--------------
contrib/postgres_fdw/connection.c | 99 ++----------------
contrib/postgres_fdw/expected/postgres_fdw.out | 134 -------------------------
contrib/postgres_fdw/sql/postgres_fdw.sql | 78 --------------
doc/src/sgml/postgres-fdw.sgml | 15 ---
src/backend/access/transam/xact.c | 28 ------
src/include/access/xact.h | 1 -
6 files changed, 8 insertions(+), 347 deletions(-)