Silence compiler warnings

Commit cfdf4dc4fc left a few unnecessary assignments, one of which
caused compiler warnings, as reported by Erik Rijkers.  Remove them all.

Discussion: https://postgr.es/m/df0dcca2025b3d90d946ecc508ca9678@xs4all.nl

contrib/pg_prewarm/autoprewarm.c

@@ -207,8 +207,6 @@ autoprewarm_main(Datum main_arg)
 	/* Periodically dump buffers until terminated. */
 	while (!got_sigterm)
 	{
-		int			rc;
-
 		/* In case of a SIGHUP, just reload the configuration. */
 		if (got_sighup)
 		{
@@ -219,7 +217,7 @@ autoprewarm_main(Datum main_arg)
 		if (autoprewarm_interval <= 0)
 		{
 			/* We're only dumping at shutdown, so just wait forever. */
-			rc = WaitLatch(&MyProc->procLatch,
+			(void) WaitLatch(&MyProc->procLatch,
 							 WL_LATCH_SET | WL_EXIT_ON_PM_DEATH,
 							 -1L,
 							 PG_WAIT_EXTENSION);
@@ -248,7 +246,7 @@ autoprewarm_main(Datum main_arg)
 			}
 
 			/* Sleep until the next dump time. */
-			rc = WaitLatch(&MyProc->procLatch,
+			(void) WaitLatch(&MyProc->procLatch,
 							 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
 							 delay_in_ms,
 							 PG_WAIT_EXTENSION);

src/backend/postmaster/bgwriter.c

@@ -361,7 +361,7 @@ BackgroundWriterMain(void)
 			/* Ask for notification at next buffer allocation */
 			StrategyNotifyBgWriter(MyProc->pgprocno);
 			/* Sleep ... */
-			rc = WaitLatch(MyLatch,
+			(void) WaitLatch(MyLatch,
 							 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
 							 BgWriterDelay * HIBERNATE_FACTOR,
 							 WAIT_EVENT_BGWRITER_HIBERNATE);

src/test/modules/worker_spi/worker_spi.c

@@ -217,7 +217,6 @@ worker_spi_main(Datum main_arg)
 	while (!got_sigterm)
 	{
 		int			ret;
-		int			rc;
 
 		/*
 		 * Background workers mustn't call usleep() or any direct equivalent:
@@ -225,7 +224,7 @@ worker_spi_main(Datum main_arg)
 		 * necessary, but is awakened if postmaster dies.  That way the
 		 * background process goes away immediately in an emergency.
 		 */
-		rc = WaitLatch(MyLatch,
+		(void) WaitLatch(MyLatch,
 						 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
 						 worker_spi_naptime * 1000L,
 						 PG_WAIT_EXTENSION);