Removed _lastTime private variable, was not used.

Button.h

@@ -36,7 +36,6 @@ class Button
         uint8_t _lastState;     //previous button state
         uint8_t _changed;       //state changed since last read
         uint32_t _time;         //time of current state (all times are in ms)
-        uint32_t _lastTime;     //time of previous state
         uint32_t _lastChange;   //time of last state change
         uint32_t _dbTime;       //debounce time
 };