pgsql: Remove _PG_fini()

Remove _PG_fini()

ab02d702ef08 has removed from the backend the code able to support the
unloading of modules, because this has never worked. This removes the
last references to _PG_fini(), that could be used as a callback for
modules to manipulate the stack when unloading a library.

The test module ldap_password_func had the idea to declare it, doing
nothing. The function declaration in fmgr.h is gone.

It was left around in 2022 to avoid breaking extension code, but at this
stage there are also benefits in letting extension developers know that
keeping the unloading code is pointless and this move leads to less
maintenance.

Reviewed-by: Tom Lane, Heikki Linnakangas
Discussion: https://postgr.es/m/ZsQfi0AUJoMF6NSd@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/15c1abd97710d74672e55d53148a0606baa91d18

Modified Files
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src/include/fmgr.h | 11 +++++------
src/test/modules/ldap_password_func/ldap_password_func.c | 7 -------
2 files changed, 5 insertions(+), 13 deletions(-)