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_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/945f0581c1e730ae5f9e4f87b79cb30e54c08ea6

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(-)