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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2e937eeb93af7539f2d65abc170d1c0369d46f12

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