Refactor to remove support for "vanity" variants

[tianon]

A compelling end-result of #61 is that we should refactor/simplify the repository again to remove support for these custom "vanity" variants (#16).

[tianon]

Here's a very rough start; there's probably a lot more cleanup we can do (looping we no longer need, variability we no longer need, etc): master...infosiftr:hello-world:no-vanity
(docker-library:6ad8af5...infosiftr:397974e - permalink version)