AP_AHRS: remove check for AVR CPUs

Remove the checks for HAL_CPU_CLASS > HAL_CPU_CLASS_16 and
HAL_CPU_CLASS >= HAL_CPU_CLASS_75. Corresponding dead code will be
removed on separate commits.

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -597,7 +597,6 @@ AP_AHRS_DCM::drift_correction(float deltat)
     }
 
     //update _accel_ef_blended
-#if HAL_CPU_CLASS >= HAL_CPU_CLASS_75
     if (_ins.get_accel_count() == 2 && _ins.use_accel(0) && _ins.use_accel(1)) {
         float imu1_weight_target = _active_accel_instance == 0 ? 1.0f : 0.0f;
         // slew _imu1_weight over one second
@@ -606,9 +605,6 @@ AP_AHRS_DCM::drift_correction(float deltat)
     } else {
         _accel_ef_blended = _accel_ef[_ins.get_primary_accel()];
     }
-#else
-    _accel_ef_blended = _accel_ef[_ins.get_primary_accel()];
-#endif // HAL_CPU_CLASS >= HAL_CPU_CLASS_75
 
     // keep a sum of the deltat values, so we know how much time
     // we have integrated over

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -47,9 +47,7 @@ public:
         _last_wind_time(0),
         _last_airspeed(0.0f),
         _last_consistent_heading(0),
-#if HAL_CPU_CLASS >= HAL_CPU_CLASS_75
         _imu1_weight(0.5f),
-#endif
         _last_failure_ms(0),
         _last_startup_ms(0)
     {
@@ -200,9 +198,7 @@ private:
     // estimated wind in m/s
     Vector3f _wind;
 
-#if HAL_CPU_CLASS >= HAL_CPU_CLASS_75
     float _imu1_weight;
-#endif
 
     // last time AHRS failed in milliseconds
     uint32_t _last_failure_ms;