Use MyBackendType in more places to check what process this is

Remove IsBackgroundWorker, IsAutoVacuumLauncherProcess(),
IsAutoVacuumWorkerProcess(), and IsLogicalSlotSyncWorker() in favor of
new Am*Process() macros that use MyBackendType. For consistency with
the existing Am*Process() macros.

Reviewed-by: Andres Freund
Discussion: https://www.postgresql.org/message-id/f3ecd4cb-85ee-4e54-8278-5fabfb3a4ed0@iki.fi

src/backend/postmaster/postmaster.c

@@ -5000,9 +5000,6 @@ SubPostmasterMain(int argc, char *argv[])
 	}
 	if (strcmp(argv[1], "--forkbgworker") == 0)
 	{
-		/* do this as early as possible; in particular, before InitProcess() */
-		IsBackgroundWorker = true;
-
 		/* Restore basic shared memory pointers */
 		InitShmemAccess(UsedShmemSegAddr);