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
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9476134a38ac0df0caaa9546447a00852efbe604

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(-)