Plane: remove old, unfulfilled method declarations

ArduPlane/Plane.h

@@ -916,12 +916,6 @@ private:
     void update_fbwb_speed_height(void);
     void setup_turn_angle(void);
     bool reached_loiter_target(void);
-    bool print_buffer(char *&buf, uint16_t &buf_size, const char *fmt, ...);
-    uint16_t create_mixer(char *buf, uint16_t buf_size, const char *filename);
-    bool mix_one_channel(char *&buf, uint16_t &buf_size, uint8_t out_chan, uint8_t in_chan);
-    bool mix_two_channels(char *&buf, uint16_t &buf_size, uint8_t out_chan, uint8_t in_chan1, uint8_t in_chan2, bool left_channel);
-    bool mix_passthrough(char *&buf, uint16_t &buf_size, uint8_t out_chan, uint8_t in_chan);
-    bool mix_trim_channel(char *&buf, uint16_t &buf_size, uint8_t out_chan);
     void set_control_channels(void);
     void init_rc_in();
     void init_rc_out_main();