deleted double used comments

Marlin/MarlinSerial.cpp

@@ -133,10 +133,6 @@ int MarlinSerial::read(void) {
 }
 
 void MarlinSerial::flush() {
-  // don't reverse this or there may be problems if the RX interrupt
-  // occurs after reading the value of rx_buffer_head but before writing
-  // the value to rx_buffer_tail; the previous value of rx_buffer_head
-  // may be written to rx_buffer_tail, making it appear as if the buffer
   // don't reverse this or there may be problems if the RX interrupt
   // occurs after reading the value of rx_buffer_head but before writing
   // the value to rx_buffer_tail; the previous value of rx_buffer_head