Summary:
With 26948390, 0bd5eff8 and f0ba436c it is now possible to
unconstrain a pointer by killing the client or deactivating the window, for
example by switching the window via the TabBox.

A user should always be able to do it therefore without needing to explicitly
break the pointer constrain as in the past with the Esc key.

Therefore remove the functionality and also remove the OSD announcing it,
which was often shown at the wrong time.

Test Plan: Manually and auto test adapted.

Reviewers: #kwin, #vdg, hein

Reviewed By: hein

Subscribers: ngraham, hein, kwin

Tags: #kwin

Maniphest Tasks: T8923

Differential Revision: https://phabricator.kde.org/D15234