[3.13] gh-142991: Fix socketmodule.c build: remove _Py_FALLTHROUGH (#143514)

Fix commit 7f93669 backport which
uses _Py_FALLTHROUGH: this macro only exists in Python 3.14 and
newer.

Author: vstinner

Modules/socketmodule.c

@@ -2652,7 +2652,6 @@ getsockaddrlen(PySocketSockObject *s, socklen_t *len_ret)
 #ifdef AF_DIVERT
     case AF_DIVERT:
         /* FreeBSD divert(4) sockets use sockaddr_in: fall-through */
-       _Py_FALLTHROUGH;
 #endif /* AF_DIVERT */
 
     case AF_INET: