AP_MotorsMatrix: remove call to empty parent Init

Also rename uses of Multirotor to Multicopter

libraries/AP_Motors/AP_MotorsMatrix.cpp

@@ -27,9 +27,6 @@ extern const AP_HAL::HAL& hal;
 // Init
 void AP_MotorsMatrix::Init()
 {
-    // call parent Init function to set-up throttle curve
-    AP_Motors::Init();
-
     // setup the motors
     setup_motors();
 

libraries/AP_Motors/AP_MotorsMatrix.h

@@ -9,18 +9,18 @@
 #include <AP_Common.h>
 #include <AP_Math.h>        // ArduPilot Mega Vector/Matrix math Library
 #include <RC_Channel.h>     // RC Channel Library
-#include "AP_Motors_Multirotor.h"
+#include "AP_MotorsMulticopter.h"
 
 #define AP_MOTORS_MATRIX_YAW_FACTOR_CW   -1
 #define AP_MOTORS_MATRIX_YAW_FACTOR_CCW   1
 
 /// @class      AP_MotorsMatrix
-class AP_MotorsMatrix : public AP_Motors_Multirotor {
+class AP_MotorsMatrix : public AP_MotorsMulticopter {
 public:
 
     /// Constructor
     AP_MotorsMatrix(uint16_t loop_rate, uint16_t speed_hz = AP_MOTORS_SPEED_DEFAULT) :
-        AP_Motors_Multirotor(loop_rate, speed_hz)
+        AP_MotorsMulticopter(loop_rate, speed_hz)
     {};
 
     // init