AP_NavEKF2: remove unused quat2Tbn method

libraries/AP_NavEKF2/AP_NavEKF2_core.cpp

@@ -1502,13 +1502,6 @@ void NavEKF2_core::StoreQuatRotate(const Quaternion &deltaQuat)
     outputDataDelayed.quat = outputDataDelayed.quat*deltaQuat;
 }
 
-// calculate nav to body quaternions from body to nav rotation matrix
-void NavEKF2_core::quat2Tbn(Matrix3f &Tbn, const Quaternion &quat) const
-{
-    // Calculate the body to nav cosine matrix
-    quat.rotation_matrix(Tbn);
-}
-
 // force symmetry on the covariance matrix to prevent ill-conditioning
 void NavEKF2_core::ForceSymmetry()
 {

libraries/AP_NavEKF2/AP_NavEKF2_core.h

@@ -594,9 +594,6 @@ private:
     // Rotate the stored output quaternion history through a quaternion rotation
     void StoreQuatRotate(const Quaternion &deltaQuat);
 
-    // calculate nav to body quaternions from body to nav rotation matrix
-    void quat2Tbn(Matrix3f &Tbn, const Quaternion &quat) const;
-
     // calculate the NED earth spin vector in rad/sec
     void calcEarthRateNED(Vector3f &omega, int32_t latitude) const;