Remove unnecessary include of <sys/socket.h>

This was put here as part of a mechanical replacement of the old
"getaddrinfo.h" with <netdb.h> plus <sys/socket.h> (commit
5579388d2d).  But here, we only need netdb.h (for NI_MAXHOST), not
sys/socket.h.

Reviewed-by: Shubham Khanna <Shubham.Khanna@fujitsu.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/cff5475d-e0a9-4561-b094-794aa36bd031%40eisentraut.org

src/include/replication/walreceiver.h

@@ -13,7 +13,6 @@
 #define _WALRECEIVER_H
 
 #include <netdb.h>
-#include <sys/socket.h>
 
 #include "access/xlog.h"
 #include "access/xlogdefs.h"