Ensure that user sees all the data before connection is closed

If user throttles receive by setting recv_mbuf_limit,
after the net interface reports connection as closed we must wait
for data to trickle through before disposing of it.
There can still b data in the buffers (e.g. SSL).

CL: mg: Ensure that user sees all the data before connection is closed

PUBLISHED_FROM=22be0fa368950a9fdb03cfb00febc7c0a1674b01

src/common/platforms/lwip/mg_lwip_ev_mgr.c

@@ -67,7 +67,6 @@ void mg_ev_mgr_lwip_process_signals(struct mg_mgr *mgr) {
         break;
       }
       case MG_SIG_CLOSE_CONN: {
-        nc->flags |= MG_F_SEND_AND_CLOSE;
         mg_close_conn(nc);
         break;
       }