directly free the transport, not need to close for maybe NULL

@@ -218,8 +218,6 @@ void SrsHttpClient::kbps_sample(const char* label, int64_t age)
 void SrsHttpClient::disconnect()
 {
     kbps->set_io(NULL, NULL);
-    
-    transport->close();
     srs_freep(transport);
 }
 

@@ -217,6 +217,7 @@ public:
  *      client.connect();
  *      client.write("Hello world!", 12, NULL);
  *      client.read(buf, 4096, NULL);
+ * @remark User can directly free the object, which will close the fd.
  */
 class SrsTcpClient : public ISrsProtocolReaderWriter
 {