Remove no-longer-referenced src/port/gethostname.c.

This file hasn't been part of any build since 2005, and even before that
wasn't used unless you configured --with-krb4 (and had a machine without
gethostname(2), obviously).  What's more, we haven't actually called
gethostname anywhere since then, either (except in thread_test.c, whose
testing of this function is probably pointless).  So we don't need it.

src/port/gethostname.c

-/*-------------------------------------------------------------------------
- *
- * gethostname.c
- *	  gethostname using uname
- *
- * Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
- * Portions Copyright (c) 1994, Regents of the University of California
- *
- * IDENTIFICATION
- *	  src/port/gethostname.c
- *
- *-------------------------------------------------------------------------
- */
-
-#include "c.h"
-
-#include <sys/utsname.h>
-
-int
-gethostname(char *name, int namelen)
-{
-	static struct utsname mname;
-	static int	called = 0;
-
-	if (!called)
-	{
-		called++;
-		uname(&mname);
-	}
-	strncpy(name, mname.nodename, (SYS_NMLN < namelen ? SYS_NMLN : namelen));
-
-	return 0;
-}