tool.h: Don't check for and #define PF_INET

This is correctly handled by ipaddr/ng_ipaddr.h today, and the check
in tool.h isn't required any more -- and caused errors on OpenBSD 5.0:

  In file included from ./../tool/tool.h:23:
  /usr/include/arpa/inet.h:74:
    warning: "struct in_addr" declared inside parameter list

src/tool/tool.h

@@ -19,12 +19,6 @@
 
 #include "portab.h"
 
-#ifdef HAVE_ARPA_INET_H
-# include <arpa/inet.h>
-#else
-# define PF_INET AF_INET
-#endif
-
 GLOBAL void ngt_TrimLastChr PARAMS((char *String, const char Chr ));
 
 GLOBAL void ngt_TrimStr PARAMS((char *String ));