Use C++ initialization list

This is the recommended approach for object initialization. The change
doesn't affect binary size (although in theory it could make it smaller).

Marlin/vector_3.cpp

@@ -22,19 +22,9 @@
 #ifdef ENABLE_AUTO_BED_LEVELING
 #include "vector_3.h"
 
-vector_3::vector_3()
-{
-  this->x = 0;
-  this->y = 0;
-  this->z = 0;
-}
+vector_3::vector_3() : x(0), y(0), z(0) { }
 
-vector_3::vector_3(float x, float y, float z)
-{
-	this->x = x;
-	this->y = y;
-	this->z = z;
-}
+vector_3::vector_3(float x_, float y_, float z_) : x(x_), y(y_), z(z_) { }
 
 vector_3 vector_3::cross(vector_3 left, vector_3 right)
 {