AP_NavEKF: Do not reset vertical velocity state from GPS

This fixes a vulnerability in the EKF, that allow large and persistent GPS velocity errors to cause large height errors. This occurs because after a timeout period, the velocity states are reset to the GPS. A reset of horizontal position states is necessary, however, because GPS is not being used as a height reference, the reset of vertical velocity is undesirable as it causes a rapid growth in height error if the GPS has large vertical velocity velocity errors at the reset time.