Get rid of malloc() and free() (PR#2549)

Just the minimal changes to qr_solve.cpp and .h to get rid of malloc() and free().

Saves about 656 bytes of progmem (library-code)
and 22 bytes for static variables.

Should use exactly the same amount of stack as it did before on the heap.

Marlin/Marlin_main.cpp

@@ -2643,7 +2643,8 @@ inline void gcode_G28() {
       #else // !DELTA
 
         // solve lsq problem
-        double *plane_equation_coefficients = qr_solve(abl2, 3, eqnAMatrix, eqnBVector);
+        double plane_equation_coefficients[3];
+        qr_solve(plane_equation_coefficients, abl2, 3, eqnAMatrix, eqnBVector);
 
         mean /= abl2;