Remove Client fullscreen hack

Summary:
This hack seems to be deactivated by default nowadays and I did not find an
option in the KCMs to activate it. Git blame shows some last commits from
around 2012 to the functions, but they only deal with code style.

Also even if it would be enabled, in light of Wayland we don't want some
roque XWayland client go bonkers via this hack.

This patch removes the code path in order to decrease complexity and ease
future changes to the fullscreen handling overall.

Test Plan: Manually and autotests pass.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, graesslin, kwin

Tags: #kwin

Maniphest Tasks: T11098

Differential Revision: https://phabricator.kde.org/D18157