AP_AHRS: remove unused get_expected_mag_field method

there's get_mag_field_NED on AP_AHRS....

libraries/AP_AHRS/AP_AHRS_Backend.h

@@ -292,11 +292,6 @@ public:
         return false;
     }
 
-    // returns the expected NED magnetic field
-    virtual bool get_expected_mag_field_NED(Vector3f &ret) const WARN_IF_UNUSED {
-        return false;
-    }
-
     // returns the estimated magnetic field offsets in body frame
     virtual bool get_mag_field_correction(Vector3f &ret) const WARN_IF_UNUSED {
         return false;

libraries/AP_AHRS/AP_AHRS_View.h

@@ -113,10 +113,6 @@ public:
         return ahrs.get_velocity_NED(vec);
     }
 
-    bool get_expected_mag_field_NED(Vector3f &ret) const WARN_IF_UNUSED {
-        return ahrs.get_expected_mag_field_NED(ret);
-    }
-
     bool get_relative_position_NED_home(Vector3f &vec) const WARN_IF_UNUSED {
         return ahrs.get_relative_position_NED_home(vec);
     }