AC_PID: remove dead get_ff(float target) method as per TODO comment

libraries/AC_PID/AC_PID.cpp

@@ -236,14 +236,6 @@ float AC_PID::get_ff()
     return _target * _kff;
 }
 
-// todo: remove function when it is no longer used.
-float AC_PID::get_ff(float target)
-{
-    float FF_out = (target * _kff);
-    _pid_info.FF = FF_out;
-    return FF_out;
-}
-
 void AC_PID::reset_I()
 {
     _integrator = 0;

libraries/AC_PID/AC_PID.h

@@ -50,9 +50,6 @@ public:
     float get_d() const;
     float get_ff();
 
-    // todo: remove function when it is no longer used.
-    float get_ff(float target);
-
     // reset_I - reset the integrator
     void reset_I();