hello-world: Change to always expect "Hello, World!"

see exercism/problem-specifications#544

exercises/hello-world/example.lua

@@ -1,8 +1,7 @@
 local hello_world = {}
 
-function hello_world.hello( name )
-  if not name or name == '' then name = 'world' end
-  return 'Hello, ' .. name .. '!'
+function hello_world.hello()
+  return 'Hello, World!'
 end
 
 return hello_world

exercises/hello-world/hello-world_spec.lua

@@ -1,18 +1,11 @@
 local hello_world = require('hello-world')
 
+-- Define a module named hello-world. This module should contain a single
+-- function named hello that takes no arguments and returns a string.
+
 describe('hello-world', function()
-  it('says hello world with no name', function()
+  it('says hello world', function()
     local result = hello_world.hello()
-    assert.are.equal('Hello, world!', result)
-  end)
-
-  it('says hello to Bob', function()
-    local result = hello_world.hello('Bob')
-    assert.are.equal('Hello, Bob!', result)
-  end)
-
-  it('says hello to Sally', function()
-    local result = hello_world.hello('Sally')
-    assert.are.equal('Hello, Sally!', result)
+    assert.are.equal('Hello, World!', result)
   end)
 end)