EKF: remove Ekf::KHP and move KHP to the methods where it's used

Everywhere where KHP is used, it is first completely reset, thus making
it unnecessary to keep it as a class member.

This saves 2.3KB RAM.

Stack sizes don't need changing, since there is already a function
Ekf::predictCovariance(), which needs around 3KB of stack and is called
close to where the fuse* functions are called.

EKF/airspeed_fusion.cpp

@@ -169,6 +169,7 @@ void Ekf::fuseAirspeed()
 			KH[row][23] = Kfusion[row] * H_TAS[23];
 		}
 
+		float KHP[_k_num_states][_k_num_states];
 		for (unsigned row = 0; row < _k_num_states; row++) {
 			for (unsigned column = 0; column < _k_num_states; column++) {
 				float tmp = KH[row][4] * P[4][column];