pgsql: doc: PG 16 relnotes, remove MAINTAIN item

doc: PG 16 relnotes, remove MAINTAIN item

Feature was removed by commit 957445996f.

Discussion: 957445996f

Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/86415153f47a4ad8e332262592e0a32f14f226c6

Modified Files
--------------
doc/src/sgml/release-16.sgml | 19 -------------------
1 file changed, 19 deletions(-)