pgsql: GiST: Deprecate F_TUPLES_DELETED opaque area flag.

GiST: Deprecate F_TUPLES_DELETED opaque area flag.

This flag hasn't been useful since the removal of old-style VACUUM FULL,
so remove it now (this includes removing vestigial code that
unnecessarily set the flag).

Also add test coverage of gistprunepage(). Had that test case been
available before now, the issue fixed by this commit would have been
detected by wal_consistency_checking buildfarm animals.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Reviewed-by: Michael Paquiër <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/CAH2-WznbTsQCrjmd=eSawfPqcxCjSFUkk6Qzd3z+gpNte5i03Q@mail.gmail.com
Backpatch-through: 14

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9c1e3b58bf541fca073892ddc4f81eef85067b50

Modified Files
--------------
src/backend/access/gist/gistvacuum.c | 1 -
src/backend/access/gist/gistxlog.c | 3 ---
src/include/access/gist.h | 8 ++------
src/test/regress/expected/gist.out | 26 ++++++++++++++++++++++++++
src/test/regress/sql/gist.sql | 21 +++++++++++++++++++++
5 files changed, 49 insertions(+), 10 deletions(-)