Remove unused <poll.h> header file in pqcomm.c

Hi, all

When I read the code of pqcomm.c, I find we don't use anything from <pool.h>
after commit cba5b994c99. Maybe we should remove the include instruction.

diff --git a/src/backend/libpq/pqcomm.c b/src/backend/libpq/pqcomm.c
index e5171467de1..744699ec35a 100644
--- a/src/backend/libpq/pqcomm.c
+++ b/src/backend/libpq/pqcomm.c
@@ -53,9 +53,6 @@
  */
 #include "postgres.h"

-#ifdef HAVE_POLL_H
-#include <poll.h>
-#endif
#include <signal.h>
#include <fcntl.h>
#include <grp.h>

--
Regrads,
Japin Li