Remove tx_pin_write from SoftwareSerial.h

It’s not implemented anywhere and unused.

libraries/SoftwareSerial/SoftwareSerial.h

@@ -74,7 +74,6 @@ private:
   // private methods
   inline void recv() __attribute__((__always_inline__));
   uint8_t rx_pin_read();
-  void tx_pin_write(uint8_t pin_state) __attribute__((__always_inline__));
   void setTX(uint8_t transmitPin);
   void setRX(uint8_t receivePin);
   inline void setRxIntMsk(bool enable) __attribute__((__always_inline__));