tcp: do not export tcp_twsk_purge()

After commit 1eeb5043 ("tcp/dccp: do not care about
families in inet_twsk_purge()") tcp_twsk_purge() is
no longer potentially called from a module.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_minisocks.c

@@ -403,7 +403,6 @@ void tcp_twsk_purge(struct list_head *net_exit_list)
 		}
 	}
 }
-EXPORT_SYMBOL_GPL(tcp_twsk_purge);
 
 /* Warning : This function is called without sk_listener being locked.
  * Be sure to read socket fields once, as their value could change under us.