AP_AHRS: remove AHRS_EKF_USE_ALWAYS define

This option now is passed when instantiating the code in ArduCopter, so
selecting the default value at compile time is not necessary anymore.

The motivation is to move vehicle specifc code out of the general
libraries. This patch shouldn't change behavior.

libraries/AP_AHRS/AP_AHRS.h

@@ -33,13 +33,6 @@
 
 #include <AP_OpticalFlow/AP_OpticalFlow.h>
 
-// Copter defaults to EKF on by default, all others off
-#if APM_BUILD_TYPE(APM_BUILD_ArduCopter)
-# define AHRS_EKF_USE_ALWAYS     1
-#else
-# define AHRS_EKF_USE_ALWAYS     0
-#endif
-
 #define AP_AHRS_TRIM_LIMIT 10.0f        // maximum trim angle in degrees
 #define AP_AHRS_RP_P_MIN   0.05f        // minimum value for AHRS_RP_P parameter
 #define AP_AHRS_YAW_P_MIN  0.05f        // minimum value for AHRS_YAW_P parameter

libraries/AP_AHRS/AP_AHRS_NavEKF.h

@@ -43,12 +43,7 @@ public:
 
     // Constructor
     AP_AHRS_NavEKF(AP_InertialSensor &ins, AP_Baro &baro, AP_GPS &gps, RangeFinder &rng,
-                   NavEKF &_EKF1, NavEKF2 &_EKF2,
-#if AHRS_EKF_USE_ALWAYS
-                   Flags flags = FLAG_ALWAYS_USE_EKF
-#else
-                   Flags flags = FLAG_NONE
-#endif
+                   NavEKF &_EKF1, NavEKF2 &_EKF2, Flags flags = FLAG_NONE
         ) :
         AP_AHRS_DCM(ins, baro, gps),
         EKF1(_EKF1),