Drop Workspace::sendPingToWindow()

Summary:
This change drops Workspace::sendPingToWindow() in order to better
separate X11-specific and more generic code.

Test Plan: Still able to close X11 windows.

Reviewers: KWin, apol

Reviewed By: apol

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D29472