Revert changes about warnings/errors for placeholders.

Revert commits 5609cc01c, 2ed8a8cc5, and 75d22069e until we have
a less broken idea of how this should work in parallel workers.
Per buildfarm.

Discussion: https://postgr.es/m/1640909.1640638123@sss.pgh.pa.us

contrib/sepgsql/hooks.c

@@ -455,7 +455,7 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 
-	MarkGUCPrefixReserved("sepgsql");
+	EmitWarningsOnPlaceholders("sepgsql");
 
 	/* Initialize userspace access vector cache */
 	sepgsql_avc_init();