AP_WindVane: Make the char_to_hex method a common method

libraries/AP_WindVane/AP_WindVane_NMEA.cpp

@@ -197,14 +197,3 @@ bool AP_WindVane_NMEA::decode_latest_term()
     }
     return false;
 }
-
-// return the numeric value of an ascii hex character
-int16_t AP_WindVane_NMEA::char_to_hex(char a)
-{
-    if (a >= 'A' && a <= 'F')
-        return a - 'A' + 10;
-    else if (a >= 'a' && a <= 'f')
-        return a - 'a' + 10;
-    else
-        return a - '0';
-}

libraries/AP_WindVane/AP_WindVane_NMEA.h

@@ -42,9 +42,6 @@ private:
     // decode each term
     bool decode_latest_term();
 
-    // convert from char to hex value for checksum
-    int16_t char_to_hex(char a);
-
     // latest values read in
     float _speed_ms;
     float _wind_dir_deg;