contrib/sockaddr: abandon __POSIX_VISIBLE define

Forced define of __POSIX_VISIBLE is no longer needed. Default
__POSIX_VISIBLE has been at least 200112 on all supported platforms
for many years. In fact, the default is much higher today.

src/contrib/sockaddr.h

@@ -16,14 +16,6 @@
 
 #pragma once
 
-/* BSD IPv6 */
-#ifndef __POSIX_VISIBLE
- #define __POSIX_VISIBLE 200112
- #ifdef __OpenBSD__
-  #define _BSD_SOURCE	/* If __POSIX_VISIBLE is defined, OpenBSD needs this. */
- #endif
-#endif
-
 #include <stdbool.h>
 #include <sys/socket.h>
 #include <sys/types.h>