AP_MotorsMatrix: remove unused _num_motors

Saves 1 byte of RAM

libraries/AP_Motors/AP_MotorsMatrix.cpp

@@ -377,7 +377,6 @@ void AP_MotorsMatrix::add_motor_raw(int8_t motor_num, float roll_fac, float pitc
         // increment number of motors if this motor is being newly motor_enabled
         if( !motor_enabled[motor_num] ) {
             motor_enabled[motor_num] = true;
-            _num_motors++;
         }
 
         // set roll, pitch, thottle factors and opposite motor (for stability patch)
@@ -411,11 +410,6 @@ void AP_MotorsMatrix::remove_motor(int8_t motor_num)
 {
     // ensure valid motor number is provided
     if( motor_num >= 0 && motor_num < AP_MOTORS_MAX_NUM_MOTORS ) {
-
-        // if the motor was enabled decrement the number of motors
-        if( motor_enabled[motor_num] )
-            _num_motors--;
-
         // disable the motor, set all factors to zero
         motor_enabled[motor_num] = false;
         _roll_factor[motor_num] = 0;
@@ -430,5 +424,4 @@ void AP_MotorsMatrix::remove_all_motors()
     for( int8_t i=0; i<AP_MOTORS_MAX_NUM_MOTORS; i++ ) {
         remove_motor(i);
     }
-    _num_motors = 0;
 }

libraries/AP_Motors/AP_MotorsMatrix.h

@@ -22,9 +22,8 @@ public:
 
     /// Constructor
     AP_MotorsMatrix( RC_Channel* rc_roll, RC_Channel* rc_pitch, RC_Channel* rc_throttle, RC_Channel* rc_yaw, uint16_t speed_hz = AP_MOTORS_SPEED_DEFAULT) :
-        AP_Motors(rc_roll, rc_pitch, rc_throttle, rc_yaw, speed_hz),
-        _num_motors(0) {
-    };
+        AP_Motors(rc_roll, rc_pitch, rc_throttle, rc_yaw, speed_hz)
+    {};
 
     // init
     virtual void        Init();
@@ -67,7 +66,6 @@ protected:
     // add_motor using raw roll, pitch, throttle and yaw factors
     void                add_motor_raw(int8_t motor_num, float roll_fac, float pitch_fac, float yaw_fac, uint8_t testing_order);
 
-    int8_t              _num_motors; // not a very useful variable as you really need to check the motor_enabled array to see which motors are enabled
     float               _roll_factor[AP_MOTORS_MAX_NUM_MOTORS]; // each motors contribution to roll
     float               _pitch_factor[AP_MOTORS_MAX_NUM_MOTORS]; // each motors contribution to pitch
     float               _yaw_factor[AP_MOTORS_MAX_NUM_MOTORS];  // each motors contribution to yaw (normally 1 or -1)