pgsql: doc: revert "xreflabel" used for PL/Python & libpq chapters

doc:  revert "xreflabel" used for PL/Python & libpq chapters

This reverts d8aa21b74ff, which was added for the PG 18 release notes,
and adjusts the PG 18 release notes for this change.  This is necessary
since the "xreflabel" affected other references to these chapters.

Reported-by: Robert Treat

Author: Robert Treat

Discussion: https://postgr.es/m/CABV9wwNEZDdp5QtrW5ut0H+MOf6U1PvrqBqmgSTgcixqk+Q73A@mail.gmail.com

Backpatch-through: 18

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/85aedc67e9674b7fdbf7627bbb54ed5b741880c5

Modified Files
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doc/src/sgml/libpq.sgml      | 2 +-
doc/src/sgml/plpython.sgml   | 2 +-
doc/src/sgml/release-18.sgml | 6 +++---
3 files changed, 5 insertions(+), 5 deletions(-)