Fix throttle angle correction when smoothing throttle; reduce processing overhead

Change the logic to not modify rcCommand directly and instead apply the additional throttle directly in the mixer.

Also move the logic to the attitude task instead of having it calculate in the PID loop. The logic relies on an angle that's only updated in the attitude task so there was no point in running the calculation every PID loop.

src/test/unit/flight_imu_unittest.cc

@@ -243,4 +243,5 @@ void performBaroCalibrationCycle(void) {}
 int32_t baroCalculateAltitude(void) { return 0; }
 bool gyroGetAccumulationAverage(float *) { return false; }
 bool accGetAccumulationAverage(float *) { return false; }
+void mixerSetThrottleAngleCorrection(int) {};
 }