Remove unused code and migrate to CMake build system.

cpp/classes/ClassA.cpp

@@ -0,0 +1,11 @@
+#include "ClassA.h"
+
+#include <iostream>
+
+void ClassA::Print() {
+    std::cout << "ClassA" << std::endl;
+}
+
+int ClassA::CompareTo(IComparable *other) {
+    return 0;
+}