Remove X clipboard sync helper and rename its autotest

Summary:
With the gneric X selections infrastructure and clipboard support the X
clipboard sync helper utility can be removed. Also rename its autotest as it
tests the inner workings of the new mechanism since this mechanism was
introduced.

Test Plan: Autotest still passes under new name.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15063