Remove ProxySocket's sent_bufsize field.

I just spotted that it was set once and never read.

proxy.c

@@ -238,7 +238,6 @@ static void plug_proxy_sent (Plug *p, int bufsize)
     ProxySocket *ps = container_of(p, ProxySocket, plugimpl);
 
     if (ps->state != PROXY_STATE_ACTIVE) {
-	ps->sent_bufsize = bufsize;
 	ps->negotiate(ps, PROXY_CHANGE_SENT);
 	return;
     }

proxy.h

@@ -71,9 +71,6 @@ struct ProxySocket {
     const char *receive_data;
     int receive_len;
 
-    /* sent */
-    int sent_bufsize;
-
     /* accepting */
     accept_fn_t accepting_constructor;
     accept_ctx_t accepting_ctx;