pgsql: Remove promote_trigger_file.

Remove promote_trigger_file.

Previously, an idle startup (recovery) process would wake up every 5
seconds to have a chance to poll for promote_trigger_file, even if that
GUC was not configured. That promotion triggering mechanism was
effectively superseded by pg_ctl promote and pg_promote() a long time
ago. There probably aren't many users left and it's very easy to change
to the modern mechanisms, so we agreed to remove the feature.

This is part of a campaign to reduce wakeups on idle systems.

Author: Simon Riggs <simon(dot)riggs(at)enterprisedb(dot)com>
Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Reviewed-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Reviewed-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Ian Lawrence Barwick <barwick(at)gmail(dot)com>
Discussion: https://postgr.es/m/CANbhV-FsjnzVOQGBpQ589%3DnWuL1Ex0Ykn74Nh1hEjp2usZSR5g%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cd4329d9393f84dce34f0bd2dd936adc8ffaa213

Modified Files
--------------
.../sgml/appendix-obsolete-recovery-config.sgml | 13 ++---
doc/src/sgml/config.sgml | 18 -------
doc/src/sgml/high-availability.sgml | 22 +++------
src/backend/access/transam/xlogrecovery.c | 57 ++++++----------------
src/backend/utils/misc/guc_tables.c | 10 ----
src/backend/utils/misc/postgresql.conf.sample | 1 -
src/include/access/xlogrecovery.h | 1 -
7 files changed, 29 insertions(+), 93 deletions(-)