Remove contrib/chkpass

Remove contrib/chkpass

The recent addition of a test suite for this module revealed a few
problems.  It uses a crypt() method that is no longer considered secure
and doesn't work anymore on some platforms.  Using a volatile input
function violates internal sanity check assumptions and leads to
failures on the build farm.

So this module is neither a usable security tool nor a good example for
an extension.  No one wanted to argue for keeping or improving it, so
remove it.

Discussion: https://www.postgresql.org/message-id/5645b0d7-cc40-6ab5-c553-292a91091ee7%402ndquadrant.com