AP_Compass: remove get_{field,offsets}_milligauss() functions

Those functions are not being used anymore.

libraries/AP_Compass/Compass.h

@@ -121,8 +121,6 @@ public:
     /// Return the current field as a Vector3f
     const Vector3f &get_field(uint8_t i) const { return _state[i].field * _state[i].milligauss_ratio; }
     const Vector3f &get_field(void) const { return get_field(get_primary()); }
-    const Vector3f get_field_milligauss(uint8_t i) const { return get_field(i); }
-    const Vector3f get_field_milligauss(void) const { return get_field_milligauss(get_primary()); }
 
     // raw/unfiltered measurement interface
     uint32_t raw_meas_time_us(uint8_t i) const { return _state[i].raw_meas_time_us; }
@@ -184,8 +182,6 @@ public:
     ///
     const Vector3f &get_offsets(uint8_t i) const { return _state[i].offset * _state[i].milligauss_ratio; }
     const Vector3f &get_offsets(void) const { return get_offsets(get_primary()); }
-    const Vector3f get_offsets_milligauss(uint8_t i) const { return get_offsets(i); }
-    const Vector3f get_offsets_milligauss(void) const { return get_offsets_milligauss(get_primary()); }
 
     /// Sets the initial location used to get declination
     ///