Remove tracking of whether write() was called.

Tracking _written was required on AVR devices to work around a hardware
limitation when implementing flush().  The ESP8266 doesn't have the same
issue, so we can remove the tracking and make write() more lightweight.

The cost is a minor increase in work done in flush() when write() was not
previously called, but this should be much rarer than individual character
writes.

cores/esp8266/HardwareSerial.h

@@ -117,7 +117,6 @@ class HardwareSerial: public Stream {
         uart_t* _uart;
         cbuf* _tx_buffer;
         cbuf* _rx_buffer;
-        bool _written;
 };
 
 extern HardwareSerial Serial;