Remove sock.h old compatibility code. Shouldn't be needed anymore.

(This provided FD_ZERO and such if the OS/libc did not provide it)

src/s_dispatch.c

@@ -55,7 +55,6 @@
 #include <signal.h>
 #include <fcntl.h>
 
-#include "sock.h"               /* If FD_ZERO isn't define up to this point,  */
 #include <string.h>
 #include "proto.h"
                         /* define it (BSD4.2 needs this) */