Remove hello-world code.

With the cumsum example the pure 'hello world' example has
become superfluous.

src/hello/__init__.py

-from ._hello import hello

src/hello/hello.cpp

-#include <iostream>
-#include <pybind11/pybind11.h>
-
-namespace py = pybind11;
-
-void hello() {
-    std::cout << "Hello, World!" << std::endl;
-}
-
-PYBIND11_MODULE(_hello, m) {
-    m.doc() = "Package to say hi";
-    m.def("hello", &hello, "Prints \"Hello, World!\"");
-}