AP_Math: remove unused angle method

libraries/AP_Math/vector2.h

@@ -95,9 +95,6 @@ struct Vector2
     // returns 0 if the vectors are parallel, and M_PI if they are antiparallel
     float angle(const Vector2<T> &v2) const;
 
-    // computes the angle in radians between the origin and this vector
-    T angle(void) const;
-
     // check if any elements are NAN
     bool is_nan(void) const;