Remove the firstreading variable

By cleverly setting _lastreadtime in begin() to make sure that it looks
like the last read was alrady 2000 ms ago, even on startup when millis()
still returns 0, there is no longer a need to keep a separate
firstreading variable, saving a byte of memory and a bit of code.

DHT.h

@@ -50,7 +50,6 @@ class DHT {
   uint8_t data[6];
   uint8_t _pin, _type, _bit, _port;
   uint32_t _lastreadtime, _maxcycles;
-  bool _firstreading;
   bool _lastresult;
 
   uint32_t expectPulse(bool level);