pgsql: doc: Remove links to further reading from pgcrypto docs

doc: Remove links to further reading from pgcrypto docs

The pgcrypto docs contained a set of links for useful reading and
technical references. These sets of links were however not actively
curated and had stale content and dead links. Rather than investing
time into maintining these, this removes them altogether since there
are lots of resources online which are actively maintained.

Backpatch to all supported versions since these links have been in
the docs for a long time.

Reported-by: Hanefi Onaldi <hanefi(dot)onaldi(at)microsoft(dot)com>
Reviewed-by: Magnus Hagander <magnus(at)hagander(dot)net>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/170774255387.3279713.2822272755998870925@wrigleys.postgresql.org
Backpatch-through: v12

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/078195eb618c86f9252300905f6e352f383fe7ef

Modified Files
--------------
doc/src/sgml/pgcrypto.sgml | 66 ----------------------------------------------
1 file changed, 66 deletions(-)