Remove unused ShutdownLatchSupport() function

The only caller was removed in commit 80a8f95b3b. I don't foresee
needing it any time soon, and I'm working on some big changes in this
area, so let's remove it out of the way.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/8a507fb6-df28-49d3-81a5-ede180d7f0fb@iki.fi

src/backend/storage/ipc/latch.c

@@ -360,33 +360,6 @@ InitializeLatchWaitSet(void)
 	Assert(latch_pos == LatchWaitSetLatchPos);
 }
 
-void
-ShutdownLatchSupport(void)
-{
-#if defined(WAIT_USE_POLL)
-	pqsignal(SIGURG, SIG_IGN);
-#endif
-
-	if (LatchWaitSet)
-	{
-		FreeWaitEventSet(LatchWaitSet);
-		LatchWaitSet = NULL;
-	}
-
-#if defined(WAIT_USE_SELF_PIPE)
-	close(selfpipe_readfd);
-	close(selfpipe_writefd);
-	selfpipe_readfd = -1;
-	selfpipe_writefd = -1;
-	selfpipe_owner_pid = InvalidPid;
-#endif
-
-#if defined(WAIT_USE_SIGNALFD)
-	close(signal_fd);
-	signal_fd = -1;
-#endif
-}
-
 /*
  * Initialize a process-local latch.
  */

src/include/storage/latch.h

@@ -173,7 +173,6 @@ extern void OwnLatch(Latch *latch);
 extern void DisownLatch(Latch *latch);
 extern void SetLatch(Latch *latch);
 extern void ResetLatch(Latch *latch);
-extern void ShutdownLatchSupport(void);
 
 extern WaitEventSet *CreateWaitEventSet(ResourceOwner resowner, int nevents);
 extern void FreeWaitEventSet(WaitEventSet *set);