pgsql: Doc: Remove mention of @ and ~ GiST operators

Doc: Remove mention of @ and ~ GiST operators

These operators were removed by 2f70fdb0644c in the v14 cycle but they were
accidentally left in the table of build-in operator classes. Backpatch down
to v14 where the operators where removed.

Author: Aleksander Alekseev <aleksander@timescale.com>
Reported-by: Colin Caine <cmcaine@gmail.com>
Discussion: https://postgr.es/m/CADwQTQbbr2UQ_fpbyc+8ay=RwEYgYk=TZxH3+RHDqAQfoG+EWA@mail.gmail.com
Backpatch-through: v14

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/af715a6f39ad8ab885b1fbfe63672ce0fcb2d662

Modified Files
--------------
doc/src/sgml/gist.sgml | 18 ++++++------------
1 file changed, 6 insertions(+), 12 deletions(-)