[kcheckpass] Drop the conv_legacy support
Summary:
The greeter only interacts through the binary protocol with kcheckpass.
There is no need to keep a not used "legacy" support around.
Test Plan: Kcheckpass gives error when called without -S argument.
Reviewers: Plasma
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D4780