AP_NavEKF3: remove default clause in setAidingMode

All aiding modes should be covered in the switch statement; adding the
default clause squelches a potentially useful compiler warning

libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp

@@ -367,9 +367,6 @@ void NavEKF3_core::setAidingMode()
             lastVelPassTime_ms = imuSampleTime_ms;
             lastRngBcnPassTime_ms = imuSampleTime_ms;
             break;
-
-        default:
-            break;
         }
 
         // Always reset the position and velocity when changing mode