ekf2: make filter update period configurable

- introduces new parameter EKF2_PREDICT_US to configure the filter
update period in microseconds
 - actual filter update period will be an integer multiple of IMU

src/modules/ekf2/EKF/ekf.h

@@ -344,8 +344,6 @@ private:
 		Quatf quat_change;	///< quaternion delta due to last reset - multiply pre-reset quaternion by this to get post-reset quaternion
 	} _state_reset_status{};	///< reset event monitoring structure containing velocity, position, height and yaw reset information
 
-	float _dt_ekf_avg{FILTER_UPDATE_PERIOD_S}; ///< average update rate of the ekf
-
 	Vector3f _ang_rate_delayed_raw{};	///< uncorrected angular rate vector at fusion time horizon (rad/sec)
 
 	stateSample _state{};		///< state struct of the ekf running at the delayed time horizon
@@ -387,9 +385,7 @@ private:
 	uint64_t _time_last_gps_yaw_data{0};	///< time the last GPS yaw measurement was available (uSec)
 	uint8_t _nb_gps_yaw_reset_available{0}; ///< remaining number of resets allowed before switching to another aiding source
 
-
 	Vector2f _last_known_posNE{};		///< last known local NE position vector (m)
-	float _imu_collection_time_adj{0.0f};	///< the amount of time the IMU collection needs to be advanced to meet the target set by FILTER_UPDATE_PERIOD_MS (sec)
 
 	uint64_t _time_acc_bias_check{0};	///< last time the  accel bias check passed (uSec)
 	uint64_t _delta_time_baro_us{0};	///< delta time between two consecutive delayed baro samples from the buffer (uSec)