pgsql: Revert "Add log_statement_sample_rate parameter"

Revert "Add log_statement_sample_rate parameter"

This reverts commit 88bdbd3f746049834ae3cc972e6e650586ec3c9d.

As committed, statement sampling used the existing duration threshold
(log_min_duration_statement) when decide which statements to sample.
The issue is that even the longest statements are subject to sampling,
and so may not end up logged. An improvement was proposed, introducing
a second duration threshold, but it would not be backwards compatible.
So we've decided to revert this feature - the separate threshold should
be part of the feature itself.

Discussion: https://postgr.es/m/CAFj8pRDS8tQ3Wviw9%3DAvODyUciPSrGeMhJi_WPE%2BEB8%2B4gLL-Q%40mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d5f53a8e26caf7931db6ad3939cd34d36ea54d91

Modified Files
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doc/src/sgml/config.sgml | 34 +++++----------------------
src/backend/tcop/postgres.c | 16 ++-----------
src/backend/utils/misc/guc.c | 15 +-----------
src/backend/utils/misc/postgresql.conf.sample | 12 ++++------
src/include/utils/guc.h | 1 -
5 files changed, 13 insertions(+), 65 deletions(-)