[kcheckpass] Drop all outdated/obsoleted checkpass variants

Summary:
This was discussed on the distro mailing list. All checkpass variants
which are no longer used are removed. What remains are:
 * etcshadow (used by Slackware)
 * pam (used by everybody which is not Slackware, including BSDs)

Test Plan: Pam path compiles

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5005