pgsql: Revert "Prevent panic during shutdown checkpoint"

Revert "Prevent panic during shutdown checkpoint"

This reverts commit 086221cf6b1727c2baed4703c582f657b7c5350e, which
was made to master only.

The approach implemented in the above commit has some issues. While
those could easily be fixed incrementally, doing so would make
backpatching considerably harder, so instead first revert this patch.

Discussion: https://postgr.es/m/20170602002912.tqlwn4gymzlxpvs2@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/703f148e98ecb4b299fdad403fc5a1de51220714

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 5 -
src/backend/access/transam/xlog.c | 6 --
src/backend/postmaster/postmaster.c | 7 +-
src/backend/replication/walsender.c | 143 ++++------------------------
src/include/replication/walsender.h | 1 -
src/include/replication/walsender_private.h | 3 +-
6 files changed, 24 insertions(+), 141 deletions(-)