AP_NavEKF: Fix bug preventing no GPS mode being selected externally

When on the ground it is likely the flow sensor will be returning  data that does not meet the minimum quality requirements selected.
The previous check was for the presence of valid data. This has been loosened to look for the presence of data.
When the vehicle becomes airborne, the quality of flow data normally improves as the image comes into focus.
AP_NavEKF: remove unnecessary function