[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