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