pgsql: Remove documentation of waiting restore_command.

Remove documentation of waiting restore_command.

Following the removal of pg_standby, also remove the documentation
section that describes how to write your own "waiting restore_command"
along the same lines.

Discussion: https://postgr.es/m/20201029024412.GP5380%40telsasoft.com
Reviewed-by: Michael Paquier <michael@paquier.xyz>

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5c6d184213bb9feac0800eee1a16769bec9e8d58

Modified Files
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doc/src/sgml/high-availability.sgml | 137 ------------------------------------
1 file changed, 137 deletions(-)