Sub: Remove unused pre_arm_check()

This is performed by AP_Arming now

ArduSub/AP_State.cpp

@@ -37,16 +37,6 @@ void Sub::set_auto_armed(bool b)
     }
 }
 
-// ---------------------------------------------
-
-void Sub::set_pre_arm_check(bool b)
-{
-    if (ap.pre_arm_check != b) {
-        ap.pre_arm_check = b;
-        AP_Notify::flags.pre_arm_check = b;
-    }
-}
-
 void Sub::set_motor_emergency_stop(bool b)
 {
     if (ap.motor_emergency_stop != b) {

ArduSub/ArduSub.cpp

@@ -363,7 +363,9 @@ void Sub::three_hz_loop()
 // one_hz_loop - runs at 1Hz
 void Sub::one_hz_loop()
 {
-    AP_Notify::flags.pre_arm_check = arming.pre_arm_checks(false);
+    bool arm_check = arming.pre_arm_checks(false);
+    ap.pre_arm_check = arm_check;
+    AP_Notify::flags.pre_arm_check = arm_check;
     AP_Notify::flags.pre_arm_gps_check = position_ok();
 
     if (should_log(MASK_LOG_ANY)) {

ArduSub/Sub.h

@@ -470,7 +470,6 @@ private:
     void set_home_state(enum HomeState new_home_state);
     bool home_is_set();
     void set_auto_armed(bool b);
-    void set_pre_arm_check(bool b);
     void set_motor_emergency_stop(bool b);
     float get_smoothing_gain();
     void get_pilot_desired_lean_angles(float roll_in, float pitch_in, float &roll_out, float &pitch_out, float angle_max);