Remove _PG_fini().

As explained by Julien Rouhaud in pg_stat_kcache's commit message
60944c460d:

Postgres hasn't called this function for more than a decade (even before
extensions were introduced), and version 15 officially removes it so let's get
rid of it too.

pg_query_settings.c

@@ -51,7 +51,6 @@ PG_MODULE_MAGIC;
 /* Function definitions */
 
 void _PG_init(void);
-void _PG_fini(void);
 
 /* Variables */
 
@@ -385,24 +384,3 @@ _PG_init(void)
 
  if (debug) elog(DEBUG1,"Exiting _PG_init()");
 }
-
-/*
- * Reset hooks
- */
-void
-_PG_fini(void)
-{
-
- if (debug) elog(DEBUG1,"Entering _PG_fini()");
-
- planner_hook = prevHook;
- ExecutorEnd_hook = prev_ExecutorEnd;
-
-#if PG_VERSION_NUM < 130000
- if (debug) elog(DEBUG1,"Recovering post_parse_analyze_hook");
- post_parse_analyze_hook = prev_post_parse_analyze_hook;
-#endif
-
- if (debug) elog(DEBUG1,"Exiting _PG_fini()");
-
-}