Remove obsolete comment

The comment in basebackup.c updated by 33bd4698c1 was actually
obsolete to begin with, since the symbols it was referring to haven't
existed in that header file for quite some time.  The header file is
still needed for other reasons, though, so keep the #include, just
drop the comment.

src/backend/backup/basebackup.c

@@ -16,7 +16,7 @@
 #include <unistd.h>
 #include <time.h>
 
-#include "access/xlog_internal.h"	/* for pg_backup_start/stop */
+#include "access/xlog_internal.h"
 #include "backup/backup_manifest.h"
 #include "backup/basebackup.h"
 #include "backup/basebackup_sink.h"